Regarding different tubes - my RM-9 (#121) came with Sovtek 6550's when I acquired it from the 2nd owner last August or so, and that's what I've kept in it. I've made so many other changes to the rig, though, (CDP, speakers, and now the PotInABox) that it would be difficult to nail down any sound trait that I could characterize as 6550 related.
But what I have played with some are the driver tubes. From the original RAM 6922's, to the 6DJ8, to a GrooveTube 6922, and just a couple of days ago, a pair of Tesla E88CC's.
Its probably a little early to say definively, but it seems I've got a bit more detail, a bit more air around each instrument with the new Tesla's. For instance, on a track from Dave Brubeck's Time Further Out, I noticed the rattle and decay of the standup bass notes more than I'd heard before.
But one thing I've learned about this hobby is that most tweaks render pretty subtle changes, and it may take some time for the improvements or detriments to reveal themselves. For now, I think the Tesla's are a positive.
I do have an octet of TungSol 6550's on the way. It will be interesting to see what happens when they're installed.