What kind of tubes? SET? Was it designed for high impedance headphones? Just general facts... not familiar with the amp at all...
The amp was tuned with Senn 600s so the Senn synergy is undeniable; although this amp is awesome with whatever I headphone I use .... especially my woodied Alessandros. This amp wil drive the ultra low sensitivity/ 150 ohm Yamaha YH-1 orthodynamics to ear bleeding levels. I have been using either a 6GU7 + adapter or a 6F8G (6SN7GT sub with an exposed anode in the pic) for gain along with whatever pair of matched 6BX7G output tubes I come across. This Supra has the most explosive dynamics you can imagine and as a routine Grados user ...trust me when I say there is no Senn veil whatsoever.
I actually have a second almost identical Supra that uses the same gain tube but with the even more powerful 6AS7G/ 6080 output tube .... which are higher current lower voltage tubes (150 volts) that drive low impedance Grados mercilessly.