Yes there are some strange exclusions from the M-CDT's spec sheet.
However your choice of CD may depend most on how you see the rest of your system being upgraded. If you're very happy with the sound I wouldn't change it too much! There are many people who've chased the upgrading rainbow and spent lots of money only to find they have a sound they like less than before. You have been warned!
The M-DAC is a very well respected piece of kit and I would imagine your RX1's are very capable speakers (I've previously owned the MA Radius R270HD's which share the tweeters of the RX1 and my current GX50's share similar midrange drivers, so I can vaguely imagine how good they must be!)
So the way I'm thinking is that a future upgrade of most benefit would be a more capable power amp to provide better bass grip of your RX1's. Again I suppose Audiolab's 8200P would be an obvious choice (depending on your budget), albeit a full width one. May I ask how do you find the bass from the M-PWR/RX1's? My own preference tends to be for a fast and tight bass; I don't actually like too much or too deep a bass as it seems to get in the way of the music rather than complementing it.
Have you considered used at all? If you were considering Cyrus, there are usually loads of CD players on Ebay to choose from. As long as you were using the digital output it ought to be ok with your system, although I'd not consider partnering the Cyrus house sound with RX1's if using the analogue output.
Sorry, waffling now, but hope that gives you some ideas. Other half width CD players include Musical Fidelity's M1 CDT (transport only), Leema Elements CD transport & the top-loading (and very well respected) Rega Apollo R. Although you'd probably want to use the Apollo's internal DAC, which your current system doesn't permit.
Before upgrading, just demo demo demo and take your RX1's with you.