The second attempt was copying my files to a buddy's laptop and using the same cable connecting the laptop's audio out to an RCA input on the receiver. Both of these configurations yielded very poor sound quality, especially at the high-end frequencies. It could be several things. One is a defective audio cable. The other is too much compression in your MP3 files.
High frequencies are particularly bad if a lot of compression is used.
So, try re-recording some MP3s with varying levels of compression until you get a satisfactory result. Remember that your receiver will be less forgiving of low fidelity than a PC audio system. I currently have a set of ancient BIC Venturis which I'm using as my mains, with a pair of Bose 501s for my surrounds, and a fairly inexpensive Kenwood center channel. I'm using Kimber speaker cable and Monster interconnects. I presume that the DAC in the DSP-A1 is better than what is offered in these components, but I'm not sure how clean and stable the original signal is going to be.