I have no major differences of opinions of the various tubes types, but as JaFox noted the JJ was (still is?) very unreliable. In fact of the new production tubes the only ones I have not experienced major failures with yet are the Svet 6550's (my primary tube of choice when combined in my amps with EI 6DJ8's). In fact one of my mono's is in the shop right now - I tried out a new set of KT88's - boom! - tube, resistor, and fuse went south. So did $150. I've got to learn how to solder.
6550 to KT88
I switched out my EH6550's to JJ KT88's yesterday in my Rogue M120's, quite a difference from the get go...much better bass, sound stage is wide/deeper and overall I like the way they reproduce the music better... so now I'm wondering if the KT90's will be any better? Anyone go from the 88's to the 90's and stay with the 90's? Thanks for your comments.