Help. System sounds thin and bright or harsh

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. 

I’m also using Puritan PSM156 and I like the sound of my Pass Labs amp being plugged directly into the wall outlet as well. Could be system compatibility when it comes to how all the component interact with each other (input/output impedance, gain, input sensitivity). 

@cymivka - I have x250.8 and psm156. They work beautifully. If you're looking for preamp, I recommend LTA MicroZOTL preamp. I just got it and sounds wonderful with Pass and Falcon Ls3a/5, which tells me it would also work well with harbeth. 

If you are using any rigid footers like cones, spikes, Rollerblocks, try removing them and replace with something pliant. If your issue is resonance related this should tell you. 

A fix no one has mentioned yet.

I've had the same problem over the last few years. Most of my listening time is at night from 11PM to 2AM. Supposed to be the best time to listen. System would start off great and get progressively brighter and thinner. Tried everything, amps, speakers, room placement, vinyl, digital, room acoustics. Nothing really helped. 

I started having a problem with the Oppo 105. Sometimes it would get stuck playing off the external hard drive. Had to be unplugged to unfreeze the gui. My VTL ST150 popped a resistor causing a tube to glow red. 

I joined the free Orchard Audio Starkrimson amp tour on the Hoffman website. The sound was so nice I had Leo build me an amp. New amp arrived and it wouldn't play! It automatically went into protection mode. The voltage was too high. As an experiment ran the amp with 300ft of 16ga extension cords. Wanted to drop the voltage. The amp played!

I now have the amp and Oppo plugged into a 20 amp variac set to 110 volts. Sounds nice throughout the night and, fingers crossed, the Oppo hasn't messed up since. Check your voltage. Mine steadily increases later at night. Funny thing, my big wire dedicated ac lines actually made the problem worse. Normal house circuits had more voltage drop and sounded better.

Loving the system and new amp.

Good luck,

aldnorab