Is it isolated to the sub amps? Have you tried detaching the inputs to the PPI amp? Try inserting muting plugs to the inputs of the PPI (single RCA connectors with the ground & signal shorted, It's an input and wont hurt the amp). Doing this at each stage will tell you where the problem lies.
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-- Murray I What part of 'no' don't you understand? Is it the 'N', or the 'Zero'?