guy did you already ship it? we had this dude buy a 880 from stereo1plus a while back and he had the same problem just a horrible screech, previous head didn't do it, and it only did it when it was on, replaced the unit with another 880, and it was perfect.
pioneer and thier ground loop isolation problems : /
i think you got the 1/1,000,000 problem with both units.
poor luck man, you'll definately recieve a new unit, at least thats what pioneer did when we sent back the bad 880 we had.