don't forget that some of the early model Mk2 sleds had a wiring fault that means that the Ground wire is not wired at all [Ground is on pin 5 of the serial connector].

All Empeg sleds earlier than about serial#1050 have this wiring fault.
[its hard to tell exactly as the sleds aren't numbered].

Try connecting a ground wire from your devices ground [probably the metal case if it has one] to the sled somewhere and see it helps any.