My amp modded by Dave has JJ e34l power tubes in it. Heck it is the 1st marshall amp I have ever had that ran them and I love it just like it is. I am a SED power tube guy normally but I have to give credit here. I am not touching this amp tube wise. It sounds great! I would think if you like the amp the JJ's would sound warmer and more musical so to speak and if you feel the need for more BITE with the amp or a touch more edge as they say then go with the SED's.
I guess the mods make it different than whem the amp is stock. I don't run JJpower tubes normally because the 4 other amps I ever got with them in it. THE JJ'S SUCKED! I even had two of the amps with BRAND NEW JJ power tubes and had nothing but problems. So with the JJ pre's the body and over all evenness is a percieved warmth to the ear. I always thought the Sed's had a mostly even response also but had or attributed a touch of edgy quality to the amp. I do know that every so often tube makers have a bad batch of tubes produced or a larger number of the tubes fail than they would like for what ever reason. So hopefully JJ's are gonna be reliable for the time being.