I'm using a Zoom 506 along with a Behringer BDI 21.Problem is,using them separately ie one at a time,they're both fine.However,when run together,using a daisy - chain from the wall wart,a loud hum appears.The BDI has a ground lift switch but this has no effect.The annoying thing is that even with the amp volume turned right down,the hum is still there.It's not a buzz,but a HUMMMMM,and the frequency is almost exactly the same as the open 'G' string.However,it does NOT increase in volume when I turn the amp up.Any thoughts,guys and gals?