Who strum on the strings of a harp; Who invent for themselves instruments of music, like David;
"That invent to themselves instruments of music, like David..." Whatever was reprehensible in those who were condemned in this clause, it was compared to a similar reprehensible action on the part of David.  Much more is known with reference to David's sinful action than is known about the sinful actions of the class Amos rebuked with these words; and a little further attention to what David did, and particularly to the action mentioned here, should give the clue to what the lords of Samaria were guilty of.  Some things can be ruled out at once.  It is not a sin to invent instruments of music, or anything else, so that could not be the thing in focus here.  However, it was sinful for David to introduce, organize, and maintain the use of mechanical instruments in the worship of God, a fact clearly discernible in  Amos 5:23.  The explanation usually offered on this interesting verse is given thus by Keil: