What I did for adding a sub to my non-DSP E53 was to source an OE subwoofer enclosure from eBay and mount it into the OE location. It took a little bit of rewiring and termination work to go from this:
to this:
and connected that up to my separate amp that is located in the spare tire well
To my ears this has been an excellent subwoofer sound added right where the OE placement was located.
Mike