I plug the two jack outputs from the MX into a Behringer Xenyx Q1002USB desk which is not noisy with any other instrument. There's a fairly quiet, low pitched mains buzz on that mixer channel only, probably 50Hz (UK). When I plug in the USB to my laptop, into which is also plugged the USB output from my Steinberg UR22II audio interface, there's some hiss and a louder, higher-pitched buzz. Disconnecting the Steinberg or the MX removes the hiss & high hum but not the quieter hum.
Some kind of earth loop? How do I get rid of it?