I have a Bryston 4B-ST which I use with a Bryston SP-1 (which essentially uses the BP-25 circuitry). I found that the Bryston gear is sufficiently revealing that the interconnects are very important. I recently replaced the IC's I was using with Alpha-Core Goertz TQ2's and found they made a substantial improvement: cleaner, more extended treble that is sweeter and less "edgy"; and a bit more transparency and tonal accuracy in the mid-range.
By all means, try a tube preamp with the 4B-ST, but I would also suggest you try a pair of Goertz TQ2 interconnects -- I think you will be pleased with the improvement in your system's sound. There was a pair of 1.0 meter TQ's for sale here on A-gon last week for $110, so you might see if they are still available. If not, you can buy a new pair of TQ2's direct from Alpha-Core for $186.