i had el34's in one with no problems. but i didnt mess with the power grid at all. the amp sounded....brasher...and mushier to me.
do a search in this forum. use "yeti" and "tourmaster tubes" or something like that.
you'll find a shit ton of info, probably more than you'll ever need regarding preamp tubes in this amp.
NOS RFT or amperex small plate sound best in v1/v2. chinese 12ax7b silver dragons are also good in 1/2/3/4.
v5 i have a jj ecc83s short plate.it does something to the contour in the od1/od2 channels that no other tube does, at least to my ears...it adds more depth and makes the contour knob more useful....v6 is reverb i think, and i currently have an electroharmonix 12ax7..but i also really love the depth a mullard cv4024 gives you. in v7, i'll tell you, use at your own risk a 12bz7(actually a higher gain tube than a 12ax7 hi grade)...ive been doing it for a while and its a kick as little trick for built in boost...(effects loop..patch a cable across the out/in...adjust levels to suit...built in boost, footswitchable...with a 12bz7, you get a bit more useable gain, pending the tube....i like the tung sol long plates.) 12ax7's work fine in v7 for doing this too, so do 12at7's. i used the stock 12ax7 groove tube here for a long time..
take it for what it's worth, i arrived at these combinations after a ridiculous amount of countless hours testing tubes with the chassis on my bench..pulling tubes, comparing, recording, etc.
when i say countless, i mean like.....probably 300 hours.
while the stock groove tube 5881's are fine, they also get a bit mushy after higher volume playing for an extended period of time, esp if you're going to use the power grid settings in the lower wattage.
personally, i love the ruby 6l6 quartet from guitar center for 50 bux. for high gain, u just can't beat them, as they dont break up as much at high volumes and stay pretty stiff feeling too...they get a lil soft though.
