tube amps 101

Rogue Audio M-120 Magnums...first rate in every way, I own a pair.

Dave

Another possibility is to keep the 4BSST and replace the preamp with a good tube preamp like a Rogue 99 Magnum. With it you can tube roll 'til you get the sound you like. I had a 99 with my 4BST and it was warm and rich yet detailed and very dynamic with the punch of the 4B; excellent detail and articulation at the extremes with a rich, smooth midrange. I finally upgraded yet again from the 99 to a McIntosh C2200 tube preamp and paired with the 4BST and the combination almost ideal. You can probably get a used 99 Magnum for about $1500.

I'd do a tubed preamp first..to test the waters, instead of putting tubes next to a Bryston pre...
I second Dave's suggestion on the Rogues, bang for your buck/entry into tubes, and cheap! Or eventually may want to look at a VTL ST85, older mb120's, or newer mb125's..
p.s. I used to have a 4BST and 3BST with a BP5 myself...blow your mind a go tubes..

Greg

Aronov Audio tube integrated LS-960I is way below your budget as used item, and maybe even when new. You will still hear the best bass slam and clarity emulating solid state, but with pleasing musicality throughout the full frequency range, and a very natural or realistic sound, minus the bright edge. (Did this win the longest sentence contest?)