I've heard very many good things about JJ's with mesa. Contact Bob or Jay at eurotubes.com and tell them what you are looking for! They can definitely help you find the tubes for you!
I've always been a traditional 6L6 guy, but I really liked EL34's in my T-verb when I had it and I now run one of my distortion channels with EL34's on my Road King. I had JJ EL34's in my T-verb, and stock Mesa EL34's in my RK.
I'm pretty sure I want to stick with 6L6s... I tried some EL34s briefly and was not impressed. But they weren't fresh and I didn't leave them in long... maybe they deserve another try.
I'm really digging the tone, I think I just want a little more definition without a harsh top. And a little bit tighther bottom with a much fatter tone overall... if that makes sense...
I'd definately get a hold of Bob at eurotubes and tell him what you want and he'll point you in the right direction and I bet you'll love what you get.
I think Bob is a great guy and very knowledgeable... but is he really the best of tube advice? I mean, I'm not trying to start anything here, but he only sells JJ tubes. What else is he going to recommend?
Personally I love the JJ E34L's in my T-verb. They made a huge difference over the stock Mesa 6L6's when I first put them in. I also really prefer the Chinese 9th Gen (Shuguang) 12AX7's in the preamp. They can also be found as Groove Tube 12AX7-C's. They are much better IMO than the stock Sovteks.
It's been a couple of years, but I really liked the Svetlana EL34s in my T-verb. The clean headroom was gone, but the smooth/focused upper mids and highs on the clean channel was very nice for clean rock tones. The Svetlana 6L6s were also very nice. Way better than the stock Mesa tubes. I ended up keeping the Svetlana 6L6s in.