Music Reference RM9 - user comments please

What tubes were you using prior? His EL34s? (Siemens?). I take it that if you made a change it was at Roger's suggestion? How did the sound signature change? I guess I could just buy them and try.

I was running the original EL34s that the amp shipped with and the amp sounded great. Talk about long tube life! Roger's statement that you'll get up to 10,000 hours on a set of tubes is realistic based on my experience.

Roger's suggestion to go to the KT88s was because I expressed curiosity and he thinks the RAM KT88s he's selling now are some of the best. I have to agree.

The change is sound was significant, tighter bass and midrange detail that took a great amp and made it even better. Soundstage is more defined by the added detail.

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.