It sounded OK with the stock tubes. With the replacement driver tubes there is more "there", there. One of these days I'll get around to swapping out the power tubes. Right now I'm very satisfied with the sound.
As for your Senns, I suggest you try all three gain settings and use the one that places the volume control closest to 12 O'clock at your listening level with your phones. That's what I did with my current preferred phones (Monolith M-560).
Sorry for the typo in post number 3 (has been corrected). The output tubes are actually 6N6