jameskk wrote on 10/11/13 at 11:08:15:no , I think I would just stick with the 6p15ev's, going back and forth the CKC (superzen) is the best of the zen's, with the c+,being second, however I could probably take the oldest Zen amp I have and go into any stereo shop and it would hold it's own or better any amp there. I have a whole slew of EL84's if you care to try them . they just don't sound as good as the 6p15ev. Spent a lot of money rolling tubes in the zen, as have others on this site,and I'm sure they will tell you also stick with the tubes Steve sends with the amp. You'll save enough money to have a new Zen amp.
EL84s are interesting in these amps, lending a more "vintage" sound (my take/impressions). Listening to a cryo'd pair of JJs right now in my modded C amp. Makes it seem more like an lp session in comparison to the 6P15EV tubes: narrower stereo separation, bit less top, nice resonance.