There is a Bryston 4B-ST currently for sale here on A-gon. I have been using this amp for about 5 years, and have been extremely pleased with it. There is also a Bryston 8B-ST in the "For Sale" listings (it's essentially a 4-channel version of the 3B-ST), which can be used to bi-amp your speakers if you wish.
If your budget allows, you might also consider upgrading the power supply for your BP-20 preamp. The new MPS-2 power supply is quite a significant upgrade to the stock power supply in the BP-20. I recently replaced the MPS-1 power supply (for my Bryston BP-1.5 phono preamp) with the MPS-2, and found that the MPS-2 provided better transient response and a bit more clarity than the older power supply.
If your budget allows, you might also consider upgrading the power supply for your BP-20 preamp. The new MPS-2 power supply is quite a significant upgrade to the stock power supply in the BP-20. I recently replaced the MPS-1 power supply (for my Bryston BP-1.5 phono preamp) with the MPS-2, and found that the MPS-2 provided better transient response and a bit more clarity than the older power supply.