Good info on the aftermarket parts, Stunt.

I get mine (mostly) from FCPEuro.com So far so good.

As for your sub project (assuming you ever want to touch the X5 again after this massive project), I had surprisingly good results dropping in ever so slightly modified (trim the mounting rings a bit) a pair of JL 6" subs. They go down pretty low -- low enough to feel the bass on London Grammar's "Hey now". Pretty musical too, picking up the low end that the HAT Imagine's leave off in the front. Not the best, but nothing to sneeze at either. Minimal modification to the stock housing.

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Kirk
Las Vegas
2016 X5 40e Mineral White/Black Dakota Leather, ZLL, ZCW, ZDA, ZDB, ZPP, multi contour seats, rear side window shades, HK stereo
2011 E90M3, 6-speed manual