AH100 Series 6 hum with rack effect

[donxiv]

So  when plugging  guitar or  Bass  direct into  the  amp it's  totally clean, but as  soon as  I plug the  Behringer V amp in any mode  direct  through or  send/return I get  mains  hum. Don't  even  need  to  turn  the  V amp  on and  it's  there. earth lift  on the  V amp  has  no  effect.

I hooked up  the  V amp to a Peavey  practice  amp and  no  hum, just  a low  level  ambient reverb.
I'm assuming  the  hum  is  down to  the  Behringer, but thought I'd  check  here to see if  there's any  known issues.

[VTypeV4]

It certainly sounds like a ground loop issue to me although I'd expect the ground lift to remedy it. I'm assuming you tried different cables and aren't running any right next to a power cable? Some of the older models (such as yours and my old TwinValve) had a ground isolator near the power switch which might help..