While trying 4CM using my HXFX and a Boogie Rectoverb head (effects loop in amp modified to series ) I noticed a hum, ground loop style coming from my amp. I tried the same setup with a Marshall JCM 900 with the same result. When I insert a Ebtech hum eliminator on one of the lines the hum goes away. If the HXFX is connected to either amps' effects loop in the common way there is no hum. I tried with a variety of cables with same results. Anyone else have this problem?