I did an outdoor gig today and couldn't solve a ground loop hum problem. Here's the particulars: * Power supplied by large generator. * DI from SWR SM-400S to PA via XLR. Hum was reduced with the amp's DI ground lifted, but never went away completely in either position. We ended up running an outboard Countryman DI off the amp's line out and that took care of the problem. Is it possible that there was a wiring or grounding problem with the generator's electrical output? The sound guy put the amps on a different outlet than the PA system, but it was all from the same source. I know that I've had to lift the amp's ground much of the time I've used it...any chance something isn't quite right with the built-in DI??