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...