It's very interesting imo to play with voltage regulation tubes on this amp.

I spent most of the past few weeks doing so and have returned to my favorite complement: three English 75C1 for the input and driver tubes. This gives me the best bass and sweetest highs and a warm midrange--just how I like it. An Amperex 90C1 gives a very open midrange in the input tube position, but there's just a bit of sterility, the warmth is gone. Of course another room, another source, other cabling, room treatments etc. will all influence how any tube will cause the amp to sound.

What a fascinating way to listen to music! This amp is the best I've ever owned.