Ok. I have an MRF-300 with an RFX-150 with some minor issues.. my question is if I hook up an RFX-250 instead of an RFX-150 will it give me rock solid narrow band RF? My remote is an MX-810. I have seen other posts about this topic saying an improvement, but is it truly making it "narrow band" or just better? Do you HAVE to have an MRF-350 to get narrow band or will the RFX-250 "upgrade" my MRF-300 to NB?

Yes, the RFX-250 or RFX-150 is the RF receiver, if you remove the RFX-150 and install an RFX-250 you have made your MRF-300, MSC-400, etc narrowband because you have changed the RF receiver. All the RFX sends out to the MRF/MSC is the command with the routing info and ID still attached. All RF reception and processing is done in the RFX, then the command is sent to the MRF/MSC.

I have 20+ MRF-300s out there that we upgraded to RFX-250s and they all work perfectly now.

That is the answer that I wanted to hear! I was 80% sure on that, but just wanted to hear it from someone else.
Thanks.