Ok, I spent most of the day over at the practice space (watching a DVD bootleg of the new Star Wars) but I was able to make a direct comparison of the GTs and JJs in my Road King.
For the record, the specific tube types are GT EL34 R versus the blue glass JJ EL34.
The difference was very apparent between tubes - much more than I even expected. I found the JJs to be superior in the amp for my applications in almost every way to the GTs. The JJs have a greatly increased frequency range over the GTs - the lows are deeper and fuller, the highs are higher, and the mids are more spaced-out and much smoother-sounding.
The JJs also seem to break up much later than the GTs did, which I didn't consider 100% favorable in channel 2 of the amp (which I have set-up using just the two EL34s for a Marshall-type Angus crunch) at first, but after increasing the channel volume and highs I was able to bring the crunch mostly back with the added benefit of dramatically increased boldness. Even still, the channels using the EL34 tubes sounded much smoother and fuller than they did before.
Even when pushed hard, the JJs are decidedly less "edgy" than the GTs were - for me, this was a bonus, but for some it may not be quite right depending on what their applications are. The deeper and rounder low-end is seriously good for me, though. Especially in Channel 4 when paired with four 6L6s (GT6L6GE's and GT6L6S's respectively). When the GTEL34's would distort, the low-end and low-mids would break up first and lose a lot of focus and definition (flubby, almost blown-speaker-ish sounding). The JJs remained solid in the low-end even when massively overdriven and beefed up the low and low-mid thump/guts/growl/whatever you want to call it so much that I "tested" them for the better part of two hours before even switching channels again.
So in short, the JJs were a success for me. I'll be leaving them in for a while.
