yeah audphile 1 it is puzzling to me as well. I found the same degradation in sound quality again with the pass labs replaced after a couple of hours listening. I have called pass labs or Reno hifi where I purchased the amp from (back in 2014) and they tell me I would hear something different if the amp was to blame, like clicking and that both channels would not likely degrade at the same pace.
I can't figure out what goes wrong...just some distortion and less meat on the bones. Felt new pre and dac fixed it but then it came back (bad sound) so I took the pass labs out of the mix and things sounded pretty good still using an Oppo bdp 105 as a transport via coax cable from DH Labs. I'm bypassing the tubed output stage using digital out.
There are only two components left the Oppo and the Harbeth 30.1 speakers. Also wondering if there some type of electrical bugaboo in the house that's only 5 years old. I am using a Puritain 156 conditioner for all components except for the pass. I felt the Puritan really made a big difference and even tried it with the pass but pass labs recommends plugging straight into a wall socket with stock cord.