I may receive death threats for this, but you might consider inserting an equalizer into your system. I would assume the Onkyo amp has a tape loop, making this possible. I have a Behringer T1951 parametric EQ on a tape loop. As long as a recording is listenable, I leave it out of the signal chain. But for those times when my taste in music and the record producer's lack thereof are in conflict, a slight reduction in the brightness range, 4-8kHz, coupled with a very subtle boost below 60Hz can often make the recording listenable, if not hifi.
My Behringer is a good choice, but I am not sure it is still in production. It also has a steep learning curve because of its flexibility. Check musician retailers like Guitar Center for availability. A less complicated route would be to pick up a graphic EQ on a site like this or that other auction site. I would look for at least ten bands per channel (12 or 14 is better), electronic level controls (these are not as subject to dust and wear-and-tear as mechanical sliders are), and the ability to store several curves in memory. The brands I like when looking at used consumer EQ units are Kenwood (I own a GE-7020 for recording), ADC, BSR and DBX. There are plenty available, just make sure you get one in top condition, as these companies aren't really around anymore to supply parts for repair (I had to replace both an ADC and a Kenwood that went bad on me). If you shop carefully, you should be able to spend less than $100, perhaps much less, for a good EQ. Get decent cables (you'll need two pairs), and you're all set.
