The Black Duo is nice. I have the US version, it's not presented me with any issues.
The Black Duo has a headphone-out jack, and volume wheel for that jack, the R and RX lack that.
Most of the complaints I've read concerning malfunctioning sound on Duo's, seems to involve the Black Duo's. I've not read quite so much abotu similar issues with the white systems (R's). Either, this is due to more of the regular Duo's being out there (And that they are a bit older), or that somethign was changed with the R's.
I've read that the R's use the same board, etc that the regular Duo uses, however, it lacks the headphone jack.
Parts may have changed with the Duo-R's. Perhaps, they used better quality parts in some areas.
In general, I've read more folsk having issues with the regular Duo, as opposed to the R and RX