On driving the HE-6 with tube amps. Regarding the Woo WA6SE I borrowed, at first I thought it was fine because it got loud without clipping, but I did eventually realize the bass was very lacking. I attribute this to the output impedance which is usually a little higher in tube amps than in solid-state or hybrid amps. I switch to the EF5 (yes, the humble EF5 which is not as powerful as the EF6 but at least has a very low output impedance) and I like the result. Not bad.
I think it's possible that the HE-6 has a very low impedance at bass frequencies and so it needs an amp with near-zero output impedance to drive it. Any significant output impedance can potentially shift the tonal balance including toward brightness.