I wonder if those are the ORIGINAL 5881's from 2000?
I don't mind those valves, but lots of people use JJ 6L6GC. I've had good results with the TungSol 7581 reissue, although I won't be getting them again since TAD (Tube Amp Doctor) are the only place that seem to stock them here and they are complete scumbag ripoff merchants. The other nice choice is KT66's, but if you go that route it's best getting some nice ones like the Gold Lion's, rather than JJ's which seem to be 6L6's in a KT66 bottle. Groove Tubes 6L6GE might be another option.
The Sovtek button base 5881 is a reliable valve due to a high maximum plate voltage rating (although it's hard to find a spec sheet).
Original 7581A valves might give you a bit more headroom in the power stage, but those are expensive now and I've never read mention of them being used. Same deal with 6N3C-E's.
Depends on what you're after. They don't make as much of a difference as changing speakers but they make a little difference and rolling valves costs a lot less. Anything will probably make a big difference if you valves in there at the moment are 13 years old, both in the preamp and power amp.
What Tony says about the level of the channel's is also correct. It's also normal to get some channel bleed from the clean channel into the gain channel, although it's usually swamped. It's also out of phase with the lead channel I think. Anyway, there is a argument that it contributes to the sound of the SLO lead channel.
It's very very subtle but really helps making it edgier. But I also use JJ E34L which make the amp a lot edgier but you can't run them in your SLO being a soldano.
The tungsol 12ax7 in V1 & V2 really makes a big difference too.
