I used to own a Bryston 4B ST. The input imedance is 50 kOhms, which by today's standards is an easy load for most preamps. My favorite preamp mated with the 4B ST was an Audible Illusions Modulus 3A, which you might find used within your budget. The Modulus 3A will flesh out the sound of the Bryston a bit over comparably priced solid state preamps, without sounding colored like some tube preamps. At the time I owned the Bryston, I also tried out Bryston, Quick Silver, Balanced Audio Technology, and various passive preamps. The Audible Illusions was the best fit of the bunch.
Alan
Alan