Music Reference RM9 - user comments please

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.

What did the Pot-in-a-Box replace? I use one and a Joule 150MKII. Not bad for $135 - is it?

Nope, not bad at all, even at the $195 for the 2-input version, which I got.
I had been feeding the Shanling CDP directly into the amp, using the built in volume control. My theory was that I was probably losing some detail and information from the CD with the digital volume, and that there would be a benefit of keeping the CDP at full volume and using the PotInBox.
Rig has been sounding very fine, though I've got fairly new Quad22L's that I think are starting to break in, and new driver tubes this week, so its hard to say what component of the great sound is due to the P.I.A.B. And the new Tung-Sol 6550's are due tomorrow.
Weren't you using a Promitheus TVC for a while? I'm interested in trying the C-cores a bit further down the road.

I was using a K&K TVC with S&B 102 MKI xformers and dual Sieden switches. An excellent piece, but I can't say I preferred it to the PiaB. I've got the Siemens EL34s in the RM9 SE, and just ordered a set of the KT88 RAMs. I've got some new SED Winged C 6550s that I am also go to try. I usually have 2 or 3 amps, but I think I might like the idea of one amp, three sets of tubes. Fortunately, I don't thing I can really tube roll (not sure why I would) the six 6bq7 driver tubes.

I've got the Siemens EL34s in the RM9 SE, and just ordered a set of the KT88 RAMs.

I hope you'll post back here after you've had a change to try the KT88s and let us know what you think.