Sound becomes less open/smeared (less detailed)

@vhond I had both the BP-25 (w/MPS-1) and the BP-19. It was a big difference.

To your question, bad capacitors can cause all kinds of weird issues. They have a definite life expectancy and when they go, they cause frustrating and inexplicable symptoms that differ from situation to situation. You can detect bad capacitors by taking off the cover and looking at them. You will see "crud" oozing from the tops, also known as "bulging capacitors".

As I already wrote down I can't see any anomalies.

For the interested people:

The schematic

Bad caps, resistors, diodes will not always have visable damaged signs. After 20 years definately have it serviced. I got back a bit more than the service cost on my 40 year old amp when I sold it 4 years later.

@vhond , it absolutely needs to be on the workbench. As mentioned above failing parts can appear perfectly intact. You have 2 choices keep using at as it fails or send it in. Last spring I bought 2 shipping crates from Rogue at $320 delivered. And then used Pirate Ship at $165 X two ways. So I was into it for $650 in just crates and shipping. I went BIG and had them upgrade while there. But like I said above they diagnosed failing diodes that LOOKED FINE and the one amp played great. Unfortunately you’re in the dynamic of “ IT IS WHAT IT IS “. Unlike most on this site I struggle to afford what I have and understand your issue. I wish you a happy resolution, respectfully Mike B. 

The Fidelity Audio website details some updates for your BP-25:
Opens with:
"The Bryston is a good discrete design pre-amplifier but is let down by a poor power supply and cheap selection of part (sic)."

It lists the parts used in the upgrade if you wish to do it yourself instead of forking out nearly $1350 ( 999 GBP)  for their service, plus freight.
(Just passing info, no connection)

https://www.fidelityaudio.co.uk/bryston-bp25-pre-amplifier-upgrade-5350-p.asp