The most important consideration is which antenna they're using. Is it the RFX-150 or RFX-250. Basically the RFX-150 is loaded with problems (interference from all kinds of things) and the RFX-250 isn't. The newer RFX-250 uses narrowband transmission and can only be used with MX-3000's manufactured on 04/01/05 or later.

I initially used an RFX-150 with the MRF-300 and had nothing but problems. Varying placement of the antenna helped a little but never really solved the problem. I tried the ferrite cores but again only minimal success. I got the newer antenna and put it right in the middle of my equipment rack to see how it performed in the worst possible location and it worked perfectly. At least 99% success. Don't screw around with the RFX-150 if their MX-3000 has the narowband transmission capability (based on the manufactured date).