Problem with system

Sorry to hear all the trouble you have had with your system. I haven't heard of a DAC compromising a preamp. 

It would be great to see your system and see all its components. It is much easier for us to make an informed estimate of possible problems. There is a place under your user ID to create a virtual system. This would be really helpful. 

Based on what you have shared, one can deduce the Bryston BP6 preamp is assumably the most likely culprit, as you’ve systematically swapped DACs, amps, speakers, and cables with no improvement.

A subtle or intermittent fault (e.g., degrading component, connection issue) might have been missed by Bryston’s initial tests. Their “nothing wrong” conclusion suggests basic functionality (power, signal pass-through, no gross distortion) was intact. But, they might not have tested for nuanced issues like dynamic range loss or subtle frequency response changes under your specific setup (e.g., load, input voltage, duration). Specifically XLR/RCA voltage changes.

However, power issues or system interactions can’t be ruled out. I'd start by removing the BP6 completely to confirm, re-test with a clean power source, and push Bryston for a more thorough evaluation. That would be step 1, and then move to whatever the outcome of that would be.

If you can share more details—exact amp model, speakers, connection types (RCA/XLR), or if bypassing the BP6 helped, it would be helpful.

Best of Luck!

Just a long shot, but check the drivers, especially the tweeters in your speakers. Perhaps one or more are blown or there is an issue with the crossover. Good luck. 

I second having your ears looked at. It may not be all of it but can be some of it.depending on your age hearing can change.an audiogram if there is minimal wax in the ear. I have cleaned alot of wax out of ears and patients were very greatful they could hear much better. I also like sounding out the room with a mic .enjoy the search.hope you find a solution soon cuz it can be frustrating.you probably changed rca xlr as well.

The volume drop along with drop in detail and dynamics was overnight.

But, it couldn’t hurt. Thx