Nice device. I had a similiar problem with my Hammond XK3. Tried changing 1/4" cables first. A simple ground lift adaptor plug solved it.
Sometimes those little things can be frustrating.
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Bill in SC --- Roland BK9 (2) Roland BK7M, Roland PK5 Pedals, Roland FP90, Roland CM30 (2), JBL Eon Ones (2) JBL 610 Monitor, Behringer Sub, EV mics, Apple iPad (2) Behringer DJ mixer