The HK system doesn't need subs, it needs mid-range clarity and decent amps -- i.e., new speakers and amps.
I spent the $600 extra on the HK, because I didn't want to tear apart my new car installing an aftermarket stereo. The HK sounds OK, nothing special, but could be worse.
The new 3-series has an optional Bose Logic 7 (?) system, which should sound better than the HK, since it's designed specifically for the interior cabin space. Bose sucks for home stereo, but since their car systems are designed specifically for the cabin, and have speakers located all over the place, their car systems sound fairly good.
If you want sound quality, don't add a sub. Having lots of bass has nothing to do with sound quality.
