<PAGE>   1
                                                                   EXHIBIT 10.14

                       AMENDMENT NO. 4 TO LOAN AGREEMENT



          THIS AMENDMENT NO. 4 TO LOAN AGREEMENT (the "Amendment") is made and
entered into this 29th day of April, 1999, by and among LaSALLE NATIONAL BANK,
a national banking association (the "Lender"), and KOSS CORPORATION, a Delaware
corporation (the "Borrower").

                                  WITNESSETH:

          WHEREAS, Borrower and Lender entered into that certain Loan Agreement
dated February 17, 1995, as amended by that certain Amendment No. 1 to Loan
Agreement dated June 15, 1995, as further amended by that certain Amendment No.
2 to Loan Agreement dated May 20, 1996, and as further amended by that certain
Amendment No. 3 to Loan Agreement dated December 31, 1997 (collectively, the
"Loan Agreement"), pursuant to which Lender agreed to provide Borrower with a
revolving line of credit up to $8,000,000.00 (the "Revolving Loan"), and with
special loans up to $2,000,000.00 (the "Special Loans"); and

          WHEREAS, Borrower has requested Lender to increase the Revolving Loan
amount to $10,000,000.00 and eliminate the Special Loans, and Lender has agreed
to do so provided, among other things, Borrower executes and delivers this
Amendment.

          NOW THEREFORE, in consideration of the premises which are
incorporated herein by this reference, and other good and valuable
consideration, the receipt and sufficiency of which is hereby acknowledged, the
parties agree as follows:

          1.     Section 2.1(A) of the Loan Agreement shall be deleted in its
entirety and replaced with the following new Section 2.1(A):

          Subject to the terms and conditions of this Agreement, on the date
          upon which all terms and conditions of the Documents have been met or
          fulfilled to the satisfaction of Lender (the "Closing Date"), the
          Lender agrees to make loans to Borrower on a revolving basis (such
          loans being herein called individually, a "Revolving Loan", and
          collectively, the "Revolving Loans") from time to time in such
          amounts as Borrower may from time to time request up to an aggregate
          amount outstanding of $10,000,000.00; provided, however, that (i)
          each borrowing by Borrower hereunder with respect to any Revolving
          Loan shall be in the aggregate principal amount of at least
          $10,000.00; (ii) the Lender's commitment to make Revolving Loans
          shall remain in effect for a period to and including November 1, 2000
          (the "Revolver Termination Date"); (iii) notwithstanding any
          provision herein to the contrary (1) upon the occurrence and
          continuance of any Event of Default, and in each such event, the
          Lender may, in its sole discretion, immediately
<PAGE>   2

          cease to make Revolving Loans; and (2) on the Revolver Termination
          Date, Borrower shall repay to the Lender all Revolving Loans, plus
          interest accrued to the date of payment; and (iv) for a period of at
          least 30 consecutive days during each fiscal year of Borrower, the
          amount of Revolving Loans outstanding shall not exceed $2,000,000.00.

          2.     Subsection 2.1(B)(a) of the Loan Agreement shall be deleted in
its entirety and replaced with the following new Subsection 2.1(B)(a):

          (a)    in no event shall total amount of Letters of Credit and
                 Revolving Loans issued and outstanding exceed $10,000,000.00;

          3.     Section 2.1(C) of the Loan Agreement shall be deleted in its
entirety and replaced with the following new Section 2.1(C):

          2.1(C) Notwithstanding anything in this Agreement to the contrary,
                 Borrower shall pay to Lender a nonrefundable nonusage fee of 15
                 basis points calculated on an annualized basis, based on the
                 average unused amount of the Revolving Loan, calculated and
                 payable on a quarterly basis.

          4.     The first paragraph of Section 2.3 of the Loan Agreement shall
be deleted in its entirety and replaced with the following new first paragraph
of Section 2.3:

          The Revolving Loans shall be evidenced by a promissory note (herein
          called the "Revolving Note") in the form attached hereto, and made a
          part hereof, as Exhibit 2.3, dated the date first above written,
          payable to the order of Lender, in the principal amount of
          $10,000,000.00.  The date and amount of each Revolving Loan made by
          the Lender and of each repayment of principal thereon received by the
          Lender shall be recorded by the Lender in the records of the Lender
          and the aggregate unpaid principal amount shown on such records shall
          be rebuttable, presumptive evidence of the principal owing and unpaid
          on such Revolving Note.  The failure to record any such amount on such
          records shall not, however, limit or otherwise affect the obligations
          of Borrower hereunder or under the Revolving Note to repay the
          principal amount of the Revolving Loans together with all interest
          accruing thereon.  The unpaid principal amount from time to time
          outstanding on the Revolving Note shall, at Borrower's choice, bear
          interest at either:  (a) the Prime Rate, adjusted as of each change of
          the Prime Rate (each Revolving Loan bearing interest at such rate a
          "Prime Rate Loan"); or (b) provided that an Event of Default has
<PAGE>   3
          not occurred and is not continuing, a rate per annum that shall be 175
          basis points in excess of the per annum rate of interest at which U.S.
          dollar deposits of an amount comparable to the amount of the Revolving
          Loan and for a period equal to the relevant Interest Period (as
          hereinafter defined) are offered generally to Lender (rounded upward
          if necessary, to the nearest 1/16 of 1.0%) in the London Interbank
          Eurodollar market at 10:00 a.m. (London time) two Business Days prior
          to the commencement of each Interest Period ("LIBOR" and each
          Revolving Loan bearing interest at such rate a "LIBOR Loan"), such
          rate to remain fixed for such Interest Period.  "Interest Period"
          shall mean one-month, two-month or three-month periods as selected
          from time to time by the Borrower by irrevocable notice (in writing,
          by telex, telegram or cable) given to Lender not less than two
          Business days prior to the first day of each respective Interest
          Period commencing on the date hereof; provided that:  (i) each such
          Interest Period may be continued upon its expiration by Borrower by
          irrevocable notice (in writing, by telex, telegram or cable) given to
          Lender not less than two Business Days prior to the expiration
          thereof, which notice shall specify that such Interest Period shall
          continue for a one-month, two month or three month period; (ii) the
          final Interest Period shall be such that its expiration occurs on or
          before the stated maturity date hereof; (iii) if for any reason the
          Borrower shall fail to select time a period, then interest on such
          LIBOR Loan shall accrue and be payable at the Prime Rate; and (iv)
          each such LIBOR Loan shall be in an amount of at least $1,000,000.00,
          and shall be in $100,000.00 increments.  "Business Day" shall mean any
          day other than a Saturday, Sunday or a day on which banks in London,
          England, and Chicago, Illinois, are required or permitted by law to
          close.

          5.   Section 2.3(A) and Section 2.3(B) of the Loan Agreement shall be
deleted in their entirety.

          6.   Section 7.1(A) of the Loan Agreement shall be deleted in its
entirety and replaced with the following new Section 7.1(A):

          (A)  Borrower shall default in the payment when due of any
               amount due and owing by Borrower to Lender under the Revolving
               Note or Letters of Credit; or

          7.   Borrower has reviewed the areas within its business and
operations which could be adversely affected by, and has developed or is
developing a program to address on a timely basis, the "Year 2000 Problem"
(that is, the risk that computer applications used by Borrower may be unable to
recognize and perform properly date-sensitive functions involving certain dates
prior to and any date on or after December 31, 1999), and has made related

<PAGE>   4
appropriate inquiry of material suppliers and vendors.  Based on such review
and program.  Borrower believes that the "Year 2000 Problem" will not have a
material adverse effect on the Borrower.  From time to time, at the request of
Lender, Borrower shall provide to Lender such updated information or
documentation as is requested regarding the status of its efforts to address
the Year 2000 problem.

         8.     Borrower shall deliver to Lender as a condition to Lender's
undertakings as provided hereunder, note amendments, a directors' consent,
secretary's certificate and such other documents as Lender shall request, each
in form and substance satisfactory to Lender and its counsel.

         9.     All references to "the Agreement" in the Loan Agreement shall
mean the Loan Agreement as amended by this Amendment.  All references to "the
Loan," "the Loans," in the Loan Agreement shall include the loan amendments
made hereunder.  All references to "the Documents" in the Loan Agreement shall
include this Amendment, the amendment to the Revolving Note and any other
instrument or document required hereunder, whether now existing or at any time
hereafter arising.  All references to "the Revolving Note" and in Loan
Agreement shall include the amendments thereto.

         10.    All of the agreements, representations, covenants and
obligations set forth in the Loan Agreement are hereby reaffirmed and restated
as of the date of this Amendment.  All representations and warranties contained
in the Loan Agreement remain true and correct as of the date of this Amendment.

         11.    Borrower agrees to pay all fees and out-of-pocket expenses of
Lender including, without limitation, outside counsel to the Lender in
connection with the preparation of this Amendment, and any and all agreements,
instruments and documents required or contemplated by this Amendment.

         12.    Except as specifically amended and modified by this Amendment:
(a) the Loan Agreement shall remain in full force and effect and is hereby
restated and incorporated herein by this reference; and (b) all terms defined
in the Loan Agreement shall have the same meanings herein as therein.

<PAGE>   5
     IN WITNESS WHEREOF, the parties hereto have caused this Amendment No. 4 to
be duly executed and delivered at Chicago, Illinois, as of the date first above
written.


LaSALLE NATIONAL BANK                     KOSS CORPORATION

By: /s/ Jim Hess                          By: /s/ Michael J. Koss
   ----------------------------              --------------------
Title: Assistant Vice President           Title: CEO/President
      -------------------------                 -----------------



                                          ATTEST:

                                          By: /s/ Richard W. Silverthorn
                                             ---------------------------------
                                          Title:  Secretary & General Counsel
                                                ------------------------------