I have an IB install on my wife's 2002 M3 (documented in bimmerforums), though not a true subwoofer setup, it is more of a mid-bass using Hybrid Audio Technology L8's in the rear deck. Bigtime improvement over the HK system "subs" (ha!) and a side benefit is more headroom in the trunk!

I'm looking at an IB in the X5 too, but only in concert with a full speaker/amplification upgrade (want to keep stock nav headunit). Still in research phase at this point.

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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