For lower powered tube amps, the best is the Coincident Technology Frankenstein's, at about $5,000. It has the strongest 8 watts per channel you're ever likely to hear, plus it is the most true to the incoming signal I have ever heard--and not by an inconsiderable margin.
At tube amp at 75 watts per channel that is almost as good as the Frankenstein is Coincident's The Dragon. But being about second best is nothing to cough about.
Dehavilland makes some good tube amps as do the much more expensive Wavac's. Einstein makes some fairly expensive top drawer mono tube amps.
Greg