I have one - I might consider selling - I have bunch of tubes - I went through 15 to 20 tubes to select these best based on Sound Quality -
Telefunken 1960 (I paid $60 + $10 ship) - these is the best sounding tube for this amp in my experience
Sylvania - rare military grade tube - made in USA?
I have shared my impressions in HE threads
If I sold - I would be selling not as much for improving SQ (as HE-5 sounds best with this amp) - it would be more into trying to get a headphone amp integrated into pre-amp / integrated amp ...
If you are interested - pls send me your offer (shipping will be extra from 11375, local pickup can be arranged)