Amplifier for Vandersteen 5A' s ?

I recently had an opportunity to listen to the Vandy 5a's and was really impressed. I currently own the Pass X-600's and think they do a remarkable job with my Magnepans, but if I buy the Vandys I would like suggestions on other amps that mate well with the speakers. Tube or SS, and preferably mono's. The budget for the amps should not exceed $6500 used. Thank you much for your suggestions.