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As far as the ground loop is concerned. Have you tried the tried and true procedure to find it?
1. Connecting the speakers to the amp only. removing all connections and interconnects, except speaker cables. Noise? yes, it is the amp. No, move on
2. Connect the pre-amp only to the amp to the speakers. noise? yes. it is the pre-amp or the cables from the pre-amp.
and so on one element at a time till you find the source.