I have the 7XD with a Leben 300XS and a Supernait 3. They both present wildly different characteristics. The Leben is a holdover from a different speaker and it sounds beautiful at low volumes but doesn't present a soundstage or have bass anything the the Supernait. If I were to ever get a tube amp with these I'd look above 40 WPC, but again that varies wildly with listening preferences and transformers, etc.
I don't know anything about the Yamaha amp you mention but in my experience you get what you pay for. As far as damping factor I'd say 4000 is a bit over the top and not needed for the 7. Never hears a Hegel amp either.