So to me it looks like the MK2 throws RF via the wire into the xover which might be a bit picky on that.

Nah, it just means you've got a ground loop. The more equipment you throw into the mix, the more of a chance you're going to get a ground loop.

Since removal of the crossover removes the noise problem, I'd say you have a difference in ground potential between the crossover and the Empeg (or perhaps another piece of equipment).

Try using heavier-gauge grounding wires for all your equipment, then grounding everything to the same point. Maybe running a ground wire from the Empeg docking sleeve to this same point would help as well. Remember that chassis are grounds as well.

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Tony Fabris