Going back to the original post. I believe the issue is the Audio Innovations s500. It's a budget integrated amp. I agree with Macca when he said "I still think it is more likely to be the equipment"
For Jazz I can understand it sounds good. But for modern music or classical which has a lot of bass response, then this amp will struggle at only 25W per channel. At anything above low to moderate listening levels this amp will begin to distort. It will sound like it is struggling.
Sources:
Turntable - Rega P2 (Modified).
Streamer - Cambridge Audio Azur 851N.
Reel to Reel - Akai GX635D.
NAS - Thecus N4200 Pro.
Amplification:
Phono Pre - QUAD Twenty Four P.
Pre Amp - QUAD QC - Twenty Four.
Power Amps - 2 x QUAD 11 Forty mono blocks.
Headphone Amp - QUAD PA - One.
Rack:
Blok Stax 400 and Blok Stax 300