I have an older 4pro and @ 2 recent gigs, I've been told by the soundman that there's a loud buzz coming from my direct out. I have them use my wet/mono output, to take advantage of the tone coming out of the BBE, in my effects loop. Now this has only happened @ 2 certain venues, and all others seem to be fine. The last sound guy was looking for a ground lift switch on the 4pro, and it doesn't seem to have one. Is this the issue ? As i've said it's only been 2 clubs out of the many I've played with these guys. Where should I start looking ?