Standard, boring answer; whichever sounds best to you...
Since I had it the wrong way round up until now, I've played my American modules with EL34s, British modules with 6L6s... I have the Twin, VX, SL2 and E'Rect modules at hand. I didn't get a bad sound! I guess it's just a matter of different flavours.
I'll do some more testing as soon as I get my rack put together!
Right now I'm running mine in stereo so I have four SED Winged-C EL34s. Sounds good. I have 6L6s and other tubes too but I don't go crazy mixing tubes. I just use the tone controls on the front of the amp.