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!