I found the Tesla, predecessor to the JJ, E34L a bit crisp in my amp and used the Svetlana EL34's which I found warm to neutral in comparison - That was in a Sonics Frontier Amp. FWIW.
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Like Newbee, I've also found the JJ tubes to be a bit brisk in my Jolida. Not overbearingly so, but noticably when compared to the Svetlanas, which are warmer but with no less detail. I've heard that the ElectroHarmonix EL34 are most impressive. I have a quad but have not opened the boxes as the Svetlanas are doing just fine right now. I'd love to hear the Mullards some day, but...$$$$!
Found the same results as above comparing tesla (JJ) e34l's to svetlana's el34's, in particular a modified stereo 70. In fact, evan prefered the svets for their open mids at the expence of the 'crisp' jj's, which would have served to help the amp.
In my experience using them in guitar amps, I found the comparisans between the two to not only be the same as using them for hi-fi, but that the svets stayed cleaner longer (didn't break up when pushed) AND was harmonically richer, and broke up smoother, evan though it is a less powerful tube than the jj. I know this is not what you will use them for, but I thought this may help being as the svet's very obviously respond better to more different variables than the jj's, such as changes in negative feedback, bias levels, gain and so forth with curcuit changes, and that seems to be an important aspect to what you are using them for.