I know this is a really obvious question, but I'll ask it anyway, as you didn't state you did these things:

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?

If you haven't tried any or all of the above, download or find your instructions, and try all of these in the order given in the manual. I find this process makes this item work!