Round 1
Although per product descriptions and user manual state that the Rebel 20 head comes with all GrooveTubes 12AX7/EL84/6V6 tubes, mine had Sovtek 7025/12AX7WA's for preamp stage and JJ EL84's and 6V6's.
Don't get me wrong, the amp is very impressive with this setup, but a man has to tweak!
On the power tubes side, I like alternatives much better than the JJ's that came with the amp. I replaced the 6V6's with a GrooveTubes GT-6V6-S duet with a max "power rating" of 10. DEFINITELY much mo' better. So far I tend to leave the power tube setting mostly all the way over to the 6V6 side or mix about 25% EL84 most of the time. The GrooveTube 6V6-S swap gives a lot more clean headroom and a warmer sound than the JJ's do.
Same deal with the EL84's. I tried both a GrooveTube EL84-R Silver Duet and a matched set of the Mullard EL84 re-issued ( http://thetubestore.com/mullardel84.html ). Either set gives you more headroom than the JJ's, but I'm going to have to do some more back and forth before I make up my mind which set to leave in, leaning towards the Mullards, but the differentiation is subtle.
As for the preamp tubes, I've used the Sovtek 12AX7WA's before, they give you a bit "darker" sound than your run of the mill 12AX7's, have a set in my Fender Blues Deluxe. I tried replacing all with GrooveTube GT-12AX7-R3's and GT-ECC83-S's and the Mullard 12AX7/ECC83 re-issues ( http://thetubestore.com/mu12ax7.html ).
Here again, I need to do some more back and forth between the GrooveTubes ECC83-S's and the Mullards before I make up my mind. Also, due to the unique design of the amp, after a lot more experimentation I may wind up using a different preamp tube for the V3 position.
More later...