Sorry, just noticed this one and could not resist...
I've had very good luck with JJ's in the Marshalls I've worked on - this after a customer demanded I use them in his JCM900s. I was impressed with their performance considering how crappy this amp is. I use them in all the 5150s I work on too. THey sound really good.
GT....where do I begin.... About 15 yrs ago I spent a ton of money, based on what I was told and advised by them, trying to find a decent KT88 for my Major. At the time their Chinese stuff was crap, their european (eastern) stuff was crap, and ALL the supposedly matched tubes they sent me were WAY off. The KT90s that I tried were just as bad. My 30yr old tired GECs with gray getters sounded infinitely better then any of the CRAP they sent me. While spending hours on the phone arguing about the condition of my amp as opposed to the crappy tubes, I bought some Svetlana (now Winged C). WOW, what a difference! Almost approaching GEC performance in tone, volume, and longevity (as I later found out). THis compared to sometimes literally hours and days for the junk they sent me. I realize this amp is not the easiest on tubes, but come on. Ratings were never exceeded, but GT's claims, real or imagined, were total BS. Bottom line, don't advertise and tell me things that just are not true. Maybe things are different now. I will NEVER darken their doorway again.
My .02
Jim