Well done review.
I just finished burning in a Classic myself, and installed it in my system today. I'm very impressed at how good it sounds, especially at its price. Mine uses the OPA 637/627 opamps, and drives my K501s as loudly as I need it to. I've got the input connected to one of my deHavilland preamp outputs, and the two seem to work harmoniously. It's kinda strange; the tubes in the preamp are almost the same size as the Classic!
The Classic doesn't make the K501s sound bass-heavy (ha!), but the bass is punchy and well-defined. Mids are detailed, and highs are extended without being edgy. The overall sound is very open and spacious, one of the things I like about the K501s.
There may be better amps out there (at a price), but the Classic is a, well, classic. Glad I got one of the last ones. Norm says he will continue to support his customers, so that potential concern is resolved.