We were using a new PA for the first time Saturday night, and I ran direct out from the back of my amp to the board. There was a noticable hum through the mains, so I ran direct out from a Sansamp Bass Driver. Same noise in the mains, even after switching the ground/phantom switch on the SABDDI. We were running late, so I just stuck a mike in front of the speaker, as there was no buzz coming through the amp. My question is, does the ground lift on the amp and the SABDDI work together? If the ground on the amp had not been changed, would there still be a hum? Any help would be appreciated.