I haven't retubed my JSX yet, so I guess I'm a liar, haha. Honestly I like how it sounds as is with the EL34's, but I also don't play metal that much anymore these days. For what I do, which is a modern style of blues-rock, it fits perfectly and I get exactly what I've been looking for. 6L6's will definitely tighten the low-end of the amp a bit, but preamp tubes have a lot to do with it as well. There is such a wide variety of preamp tubes to fit the bill for heavier styles, but to go along with them is an even wider variety of taste and preference. For me, I've always e-mailed Bob at EuroTubes and told him what I'm looking for out of any of my amps that I was retubing and he suggested a perfect setup. Perfect as in, when I got them in the mail, put them in, and turned on the amp, it was exactly what I had been wanting, heh.
The JSX is the only amp I own that doesn't have an all-JJ setup, and thats only because I haven't switched anything out yet. Both Mesas, the Engl, 5150, and both Marshalls have nothing but JJ's in them and even if I don't like the amp itself *ahem Marshall ahem* I still like the tone a lot more with the JJ's than I did before.
I've even caught Bob suggesting other tubes before besides JJ's in e-mail conversations because he knew what it was I was looking for out of the amp and in particular situations he felt that JJ's may or may not give me the desired outcome. That alone makes him a very useful contact, IMO, because he obviously isn't out to just get your money. He strikes me as being a sincere person when it comes to making sure you are getting what you want out of your amplifier(s).
~e.a
