Guys, you helped me get back to some obvious stuff I hadn't done. I have a list out in the car to address the various recommendations, which I'll post later.
But the initial test was revealing. I got out my Radio Shack digital sound meter, and grabbed one of the 6 test CDs I own - as if one wasn't enough - and played a 1Khz tone track. Measured the left and right speakers, and got about a 6dB difference, with the right being louder.
Then I swapped the the output channels on the amp by connecting the right speaker to the left output terminal and the right speaker to the left output terminal. Voila! 6dB difference with left speaker louder.
So the amplifier has been indicted, but not found guilty quite yet. Two more things to do tonight: first, going to swap the output tubes in the Ayon CD-2 to see if the tubes are the culprit, although they shouldn't be because I remember I had this issue with my Cary 303/300. If swapping the tubes doesn't work, I'll get out the RA Opus 21 I have sitting in its crate ready to sell and see if I have the same issue using that CDP. If I do, the amp heads in for a checkup! :-)
Thanks a million for all the help you guys have offered! If I had asked for help on this 18 months ago I could've saved myself literally 50 hours of work!