I'm getting a terrible noise using the 4 cable method to run my HD500 with my Mesa Mini Rectifier. My setup is guitar -> HD500 input, HD500 send -> amp input, amp send -> HD500 return, HD500 output -> amp return. There is a huge buzz that develops when the loop is engaged. The frequency of the buzz with take on a lower frequency hum if I move around the input cable to the amp (from the HD500 send), and will also change if I move the head around or rotate it 90 degrees. With the loop switched off, and putting all of my HD500 effects before the send (and thus before the amp), everything is dead quiet. But this defeats the purpose of having a loop for time based effects, which for me is absolutely necessary. Clearly there is some form of EMF shielding issue somewhere in my system. I have tried the HD500 in the same configuration with my other amp (Blackstar HT20), in the same place in the room, with no problems. I have tried different cables with the same results. I have tried different HD500 grounding and pad settings, with no benefit. All other configurations with the HD500 are acceptably quiet, until I use the 4 cable method. The issue seems to be somewhat variable, ranging from bad to worse for factors I do not yet understand. I have tried a power conditioner with no benefit as well. Any idea what may be causing this, or more importantly, what to do about it?