You can get at least one of them at Crutchfield if you're in the US. I don't know the difference between the two.

True. In the meantime I found info on the RM4XS on their site. No mention of the RM2XS though. But since I don't live in the US it doesn't really matter.

it requires building your own serial-port interface with a PIC controller and blowing an EPROM with code. Since you already have a tuner module, that's much easier

Eh...point taken.