Mitch57, I also have an MX500 and an Onkyo receiver. It's not clear to me if the model you listed (HT-R500) is the same as my TX-SR500 (one might be part of a speaker package, and the other separate, but otherwise they might be identical, but to be honest I'm not sure).
Anyway, I ran into a power discrete issue which you might want to read about in the following Discrete Hunter thread:
[Link: remotecentral.com]Though I was able to obtain 1 of 2 discrete power commands, in the end I decided to solve my discrete power command problem by always leaving the receiver on.
Oh, and BTW, I use a RS remote which I picked up on clearance recently to learn most of my commands to my MX500 using 'Advance Codes'. Haven't needed JP1 or IRClone yet (and I've completed my setup) though both look pretty cool.
BEK