I have tried this with my M13 and RM50 (use the series loop). It works, but there is a slight loss in gain/signal with this 4CM set up. Note that you need to set the bypass mode in the M13 to DSP bypass, or 4CM won't work at all.
Not sure what your issue is with the loop in the RM50. Mine was ground loop hum and I was able to fix it with this:
http://www.guitarcenter.com/Art-CoolSwitch-Pro-A-B-Y-Switch-with-Isolation-J13727-i3897185.gc?country=us¤cy=usd&source=4WWRWXMP&kwid=bingproductads-plaid^26593250368-sku^J13727@ADL4GC-adType^PLA-device^c-adid^3645655862
I had to put it between every cable that ran between the RM50 and the M13 until I found the right one. It turned out it was not just my M13 but also a Digitech GSP1101 that had the hum. This fixed everything.