SWR Combo DI hiss/hum?

[cm3ohana]

Today, the output from the XLR DI output from our Workingman 15 combo amp is now hissing/humming. The output from the speaker has no hiss/hum but when we take the XLR output from the DI output in the back of the amp to the FOH board, there's an audible hiss/hum thru the house PA. We tried switching XLR cables, plugging the amp into different outlets and power conditioners with no change. Using the same XLR cable, We plugged the bass into a BDDI into the same channel of the FOH board and no hiss/hum which ruled out the XLR cable, FOH board and power sources. So, we actually unplugged the amps power, plugged in the XLR cable into the DI output of the amp and it still hissed/hummed. I'm stumped and never had that happen before. We thought that the amp may be receiving some king of radio signal interference so we moved the amp around the stage and still the same problem. Everything seems to work fine on the amp except for the DI out. Anyone ever have this problem? Thanks