Don't really know what the JJ's are. But I was watching an Interview w/ Mark Tremonti on YT (not a fan or into/familliar with his stuff but I am interested to know about the new MT100 amp that's due out sometime soon) and he said that the 5881's in the new MT-15's were "Russian 6l6 style Military grade tubes" ( I'm guessing 6P3S-E) in the video below @ about 4;20 mins..
All I know is I'm a big fan of 63PS-E's..they're my favourite 6L6-style tubes...
For EL34's I like the Shuggy's best..
And I like the regular short plate JJ 12Ax7's best for preamp tubes. I've got them in most of my amp's. (Also have Shuguang's in a couple of them, though I often find them to be too bright/thin/harsh for my tastes)
My Brutus amp came stock with Sovtek's & the preamp tubes were perfect for that amp, nothing I swapped them out for sounded better , so they're still in there. I swapped out the stock Sovtek 5881's (sounded ok, but not great) for 6P3S-E's and that was a huge improvement to my amp in every way. The instant they went in everything just sounded "perfect"....like the whole amp was functioning in it's sweet spot.
Anyway..here @ 4:20 mins.