Which Preamp for Bryston 4BST. BP-25 or Rogue 66

I am in the same situation as yourself..looking to change from a SS BP-25 Bryston preamp to tubes.
As pointed out to me by someone in the know..you have a very good preamp amp with the BP-25 so make sure that you look for the same or better quality tubed preamp if you want to see a significant improvement.
The two tubed preamps suggested were..Rogue Audio 99 or BAT V50.(something like that)
I was very excited with the fact of changing to tubes but that statment brought me back to earth and changed my way of thinking.
So am I on the right track only looking at tubed preamps in the $2000-$3000 range and nothing less.
Sorry I didn't answer your question but hopefully we can both benefit from some further responds.

Detailed generally equals more bright.The 66 will add the warmth that makes music very enjoyable.No listner fatigue.

I have the 7B-ST's and they perform very well with a Rogue 99. I imagine the 4B and the 66 will also be a very good combo.

I'd opt for tubes.Rogue is great and Mark O'Brien is there on other side of phone to back it up if need be (real nice guy to).Other (used) pre's to maybe think of would be the Audible Illusion Modulus 3A,BAT VK3i.But Rogue would be great and I hear that Magnum upgrade is worth the money and would be just the ticket with some good NOS when you ready to tube roll.

Blue Circle BC21 is a terrific match with 3B-ST or 4B-ST.