Did you read the Manual on the MRF-300?
Did you check for interference?
Did you change the channel on the MRF to other than 0?
Did you try adjusting the line level output?
Did you try sending commands to only 1 line level output?
Did you try disabling the front IR blaster?
Yep, I did indeed perform all of the above steps ... no interference, the RF signal from MX was coming thru w/o issue (as shown by both the antenna & MRF's led); channel was set to "B"; line level adjustment didn't at all affect the direct 3.5mm cable's not working; the remote was configured so that each component's IR commands were only sent to the MRF port to which it was connected, and of course the front IR blaster was disabled ;)