@tomcarr Swap the channels left for right between the amp and preamp. If the problem moves then the amps are off the hook.
Since you tried 2 amps you probably don't have to do this test, but you do have a source which could be the problem. Swap the interconnects between it and the preamp. Play the offending tracks and see if the problem moved to the other channel.
If it did => the source has the problem
If it did not => the preamp is the problem
Good luck!