fishman rare earth blend buzzing mic?

[64cousins]

Hi All.
I got a fishman rare earth blend today, I think it has a fault but I thought I'd run it passed you fine fellows in case I've overlooked something stupid.

when I plug into my AER compact 4 there is an enormous buzzing, when I roll the blend knob over to 100% pickup and it goes away and sounds very nice.
if I blend any mic back in the loud buzzing comes back and if a touch the mic it buzzes even louder. like when you a a duff cable on your pedalboard.
I have changed guitar cable.
changed channel on the amp.
gone straight into a pa system all with the same result.
That has Got to be a fault right?