If you are using stock wiring to the doors, that might be the problem. Stock speakers are rated at?? Head units put out 20w rms or so IIRC. They weren't thinking "let's put in speaker wire that will handle 10 times what the stock head unit puts out, in case someone puts in some high power speakers" when they designed the car
It sounds like that is your problem. You can even try one channel on one door and one channel on one speaker wired directly to the door (same distance) with bigger wire. Should tell you if it's the wire size or not.
Bc2000d - jvf FI BL 15" - s800/4
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