If you are adding an amp to an X5 that has only door speakers, I personally don't think that the benefit gained by using preamp outs from the HU instead of speaker level outs is that big a deal.

A lot of the potential improvement when doing this is in noise floor and other noise issues, which subs really don't manifest. Buit using those preamp wires AND retaining the OEM amp can lead to noise issues with the OEM amp, which you want to avoid dealing with...

I'd just tap the woofer wires for the front doors for a speaker-level if your system lacks OEM subs.

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