Anytime you are talking about adjusting speakers or a sub amp,you need an o-scope so you can see where the signal is clipped.Disconnect your sub wires and insert the o-scope leads into the speaker inputs.Turn your head unit up to the volume where the signal is cut off at the top,or clipped.Then you would adjust the filters on ur sub amp to obtain the cleanest signal to the sub itself.

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'El Presidente'2003 4.4i,Ti Ag(Titanium Silver)XXXXXplicit-The X5 a.k.a X to the Fif