The E34L tube can be used in any circuit spec'ed for an EL-34 as far as I know, but I wouldn't expect any increase in actual power just from using this tube. That 15% quote probably means something like that the "L" version is capable of yielding up to 15% more power than the non-"L" version in a circuit designed to do that. Even that conclusion might be open to question, but anyhow I did find that the "L" tended to outlast the regular version from JJ in a somewhat demanding amp circuit. (But I'll also add that I thought my best results of all, both sonically and durability-wise, were with the Svetlana EL-34's.)
Anyway, if these tubes are in good new shape, reasonably well-matched, non-defective, and survived shipping OK (there can never be guarantees on any of that, even if they were supposedly pre-tested, until you actually install them and they work without problem), go ahead and enjoy them, they're fine tubes and your amp will thank you for them.
Anyway, if these tubes are in good new shape, reasonably well-matched, non-defective, and survived shipping OK (there can never be guarantees on any of that, even if they were supposedly pre-tested, until you actually install them and they work without problem), go ahead and enjoy them, they're fine tubes and your amp will thank you for them.