Is your Cinema 6 the learning version? If so, it's JP1 compatible. That's the best way to get it to control ReplayTV, plus you'll be able to make it control any other new devices you get (including the Marantz) and you can convert into a 7 device remote!
If it's the non-learning version, it can still be made JP1 compatible but it will take a little bit more work (and some detailed soldering).
If you want a remote that's ready to go, just pick one of the newer One For All or Radio Shack models. In fact, OFA has just come out with a remote especially designed for PVRs called the URC-6131, which can be had for $18 at Circuit City.
Rob
http://www.hifi-remote.com