It's funny that you should ask that now, as I feel that they have just finished their "break in" period, and I had to re-tune my RM40's to them. In short they have gone from great to awesome. The only issue I had with them before breakin was their much more "upfront" sound. A lot of the soundstage depth was gone, and the overall sound was more angular, inspite of using Mullards in the 550ex (which had been very soft sounding with these tubes before I sent it off). But the increase in dynamics and resolution were very satisfying. I figured I would just need to get used to the new sound.
Then there were several weeks where I could not really listen much, so I kept the system on and running tunes through it, even though I was busy with other things and couldn't sit in the sweet spot or even listen critically much at all.
But this past week I got a chance to sit down and re-listen and also re-tune the 40's a bit. And I gotta say that it was completely and absolutely worth getting the upgrade, the improvements are now across the board. The tonal balance and warmth of tone is at least as good as before I sent it off, while impact, bass, resolution, all have taken a big step up. So, I've gotta say I'm pretty dang happy. Considering that the "R" upgrade is now standard on new units, they go from "solid" rec's, to "no-brainer" rec's.
I know that I "should" drag some other world class amps and preamp over to do comparisons to really see how things measure up (like I have done in the past), but honestly the sound I'm getting is so good, and so satisfying, that I just do not care what other stuff sounds like. I'm too busy enjoying my music.