HPF-high pass filter. cross over function to ng 7550. 7550 pala deck mo may front, rear and sub out pa off mo na lang all the crossover switches ng amp.

ang pumipito ba sounds like huming? some signal wires may have crossed paths with power cables, or try to tighten rca wires, insulated ba kinabit or the simple kind?

make sure na ka on ang crossover ng HU mo is you used it especially yung rear, you can read this sa manual on how to do it.