Hi all
I'm a sax & EWI player using a UFX+ with MacBook Pro and Sennheiser wireless mic and IEM systems.

One show I'm involved with has started doing a lot of high-end theatre shows and I was told the other night that my rig is emitting electrical noise...not much, but enough for the theatre AV guys to be unhappy about it.  I was stunned because I've never encountered this before but the AV guys said that's likely due to many gig venues not having very low noise floors, whereas high-end theatres are very very picky about this sort of thing.

After much testing, we found that the UFX+ XLR outputs are emitting ground noise.  There's no noise from the 1/4" outputs and when the UFX+ is connected to a separate power line (as opposed to it usually being on the same powerboard in my rack as numerous other devices), the noise disappears. 

I'm not an electrical/electronics/AV guy and would love some thoughts/answers/advice re. the following:

a) Why the UFX+ XLR outputs have noise in the first place?
b) Why ALL the UFX+ outputs don't have noise (i.e. why is there only noise at the XLR outputs and not the 1/4"?)
c) Why there's noise from the XLR outputs even when there's no power to any devices in my rack, including the UFX+?
d) Apart from running separate power to the UFX+, is there any way to completely eliminate the noise instead of applying band-aid solutions such as isolator boxes etc?

Cheers in advance, guys.  I'm keen to learn about these things and would really appreciate any advice.
Gary