Tvad, I mated the 4Bst with a Audio Electronic Supply (AES) AE-3 tubed preamp a while back and what a difference tubes have on this amp. You can buy a used AES for $300-400 and this is a great start into the world of tubes.
I also inserted a pair of Acoustic Zen Matrix II IC's between amp and preamp, and these cables brought out hidden qualities in my system.
The BP25 is most likely where you should start disecting your equipment. Many reviews state that the BP25 can show how poorly a source can be( cd...phono )because it is a very neutral preamp. You might be playing a cd that was poorly engineered or produced, and by turning up the volume, you are also amplifying that same source material. A tubed preamp can give you that creamy smoothness, with the Bryston 4Bsst offering its dynamics.
good luck, kek