I retubed this summer my 400+ with JJ 6l6 and the 12x7 from Eurotubes.
imo, the sound was a little bit fuller, but it was subtle.
Nothing to be heard at gigging situation.
Last week I also had the bias adjusted by a local tech.
He said he warmed the bias a bit to 32mA to each tube.
Well, the result is impressive !
The sound is more 3D, the treble and high mids are sweeter, the low end is more controlled.
Overall, a significant fuller sound (my wife would disagree, but that's an another story
Ok, it's still sound like a 400+, bit everything seems more right. No gig at the moment, so I can't say how it will translate in live situation.
I was going to sell this amp because I found it to aggressive, but I think it is now a keeper and maybe the time to have a better cabinet pairing (running a pair of gs112)
The only negative point : 2 power tubes seem to rattle a little bit with cab vibration. Subtle but it's there at bedroom volume.
Nothing to worry at gigging volume. Jay at Eurotubes suggested me to change the position of the tube and that sometimes it can help. He also talk about tube dampers... I'll have a look at this option. Maybe a Gramma pad will help. If not, I can always put the rack on the floor and problem solved !
Question : Is it Ok to let the tube rattle : will it damage them ?