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).

Im driving my 6371's with a Manley Stingray in Ultralinear 50W. This is one heavenly matched sound! Excellent bass drive and dynamics with sweet mids and highs. Ive tube rolled a got a sound I truly love. :)

usher speakers can be driven by both solid state or tubed components but are better matched with transistor amps.
i have a pair of usher dancer series 777 with beryllium tweeter and the sound driven by a tubed 50 watts per channel integrated is good but not enough to grip and pin down the bass with authority, perhaps it would be a different story with some antique sound labs hurricannes!
you should check out pairing with the new usher electronics wich were made especialy to mate their speaker line.
r-1.5 power amp that pushes 150 watts per channel in class"A" operation,if you do not get good grip on the bass with that you never will.
their preamp is a nice little one too, with its own separate power supply, the combo just got raving reviews in
a german magazine and the power amp was tested by hi-fi-world and the verdict was that this amp was a stone cold sonic bargain!