tube amp for usher 6371

A used Cary V12 would be well within your budget and drive the Ushers easily. I bought one about a month ago and my Beethovens need about the same amount of pwer as your speaks. The sounds great and it also has balanced and single ended inputs. Also it has flexibility to switch to full or fractional ultralinear if needed. I haven't even tried ultralinear, no need.
Jeff

I heard them driven by Rogue amps at a dealer nearby and they sounded really good.

I've tried a few tube amps on the the Usher CP 6371 speakers- Prima Luna Prologue 1, Antique Sound Labs AQ1001DT, and Granite Audio 861 monoblocks. These are all push-pull and put out 30-40W. The Usher's are excellent speakers and sound pretty good with each of these, but I don't think I am getting the most out of the Usher's with these amps. My less sensitive previous speakers (Harbeth HL5) seemed better controlled by these tube amps. My guess is that it might take a little more tube power to bring out the best in the Usher's. Read somewhere that the CP series were voiced for solid state amps. I've been amazed at the Usher's bass response, image focus, and detail and dynamics at low volume using a cheap solid state integrated (Jungson 88C). It might be worth considering a tube hybid amp (BAT, Pathos, etc).