Mesa V-Twin hum

[guitar_gazza]

Hi all, I am using a Mesa Boogie V-Twin in the effects loop of a Boss GT 8. This works fine until I try to switch modes on the V-Twin with the amp control jack on the GT 8. As soon as I connect this amp control I am getting a hum that is even noticeable when a note is played. I am pretty sure it isn't an earth loop as both of these units have no earth pin on their power supply.

Any suggestions?

Any help appreciated.