I recently switched out my Tungsol KT66 tubes in a JTM45 to Sovtek 5881s.
I'm very pleased with the result- quieter, crisper @ clean, crunchier @ 10 with MV down.  Anyone have experience switching to 5881's or to 6L6GC's as alternative power tubes??

The Sovtek 5881 is a weird tube.  It's actually a Soviet military jet tube singled out by Mike Matthews because on paper it performed very much like the original 5881/6L6GB's from the '50's...but it doesn't sound the same.  15-18 years ago, there were no real 6L6GC, 6V6GT's, EL34's being made anywhere (Tesla EL34's were banned due to the Yugoslavian civil war, then the factory was bombed), so Matthews practically had a lock on the power tube market (there were poorly made Chinese 6L6GB's being made).  Anyway, it stands along apart from all the other 6L6-style tubes because it isn't from the lineage of RCA, TungSol (original), GE, etc.

The 6L6GC will have a scooped midrange compared to a KT66.  The British referred to this scooped midrange as a "midrange kink."  The KT actually stands for Kinkless Tetrode, the tube was designed to be a 6L6GC with the midrange put back in (the KT66 was designed as a hi-fi tube).  Ultimately, this extra midrange works against clean headroom and the tube gets a slightly more aggressive tone. 

The 6L6GC will offer noticeably more headroom and overall output over the 588, and a bigger, rounder, cleaner tone over the KT66, but will also have a more focused overdrive that may not seem as crunchy or as organic as a KT66.

When you were putting these different tubes in your JTM45, did you have to do something beside just change which tubes you were using??

I asked Nik when he built my JTM45 to make it available for EL34 tubes with just a bias adjustment but I wondered if it was alright to just swap tubes or not. Do you like or prefer the other power tubes over the KT66??

Also, you mentioned you have a master volume on your amp. So do I, and I've read about guys taking them out because they said that it changes the tone in a negative way. Has anybody done that and what are their experiences with the master volume on this amp??