I just swaped my JL amp into high pass to drive the highs and mids, it is the same, hiss. It is now clear to me that the noise does come from the HU, however the input gain of JL amp can be turned much lower so that the noise can be kept lower, but the alpine not. I am frustrated absolutely. Since my JL amp has just two channels it can only be used to drive subs. The BWM HU is also made by alpine why the alpine amp doesn't work together with alpine HU! I think it is true that BMW amp has noise deduction but aftermarket amps don't.
What should I do? Restall the BMW amp?