new to tube amp

I have a bryston 4bsst and bp25, sonic frontiers sfcd-1, green mountain europas. I am very impressed with the 4bsst but was contemplating moving into tube amp -- anything that is decent for about $2K or is this wishful thinking? Or should I be thinking of replacing another component?

What sort of questions should I ask when purchasing a used tube amp?

Thanks for the advice...
For your budget, I would suggest the Rogue Audio M-120 or the M-120 Magnum as one of your tube amp options. Either of these tube MonoBlock amps can usually be found used here at Audiogon for under $2,000