My 2007 550i had the code 676 Hi-Fi speakers. BMW seems to actually make at least 4 levels of audio:
Base (not on cars for exprt to US, but you will read posts from folks in Europe who have it),
Hi-Fi (yours - which is the "base" system for US-export cars),
Logic7 (code 677, HiFi system Professional DSP - adds more speakers to include a center-channel on the dash top, larger under-seat subs (8" instead of your 6") and a DSP amp, and
Individual Audio in top-end cars, a $700 option (code 752 I think...) that may also be referred to as a DIRAC - adds an even better DSP amp and speakers - seen on M3, M5 and M6. Check out Bavarian Soundwerks for some good info on the speakers as well.
For your direct comparison, be sure it is exactly that: check your bass and treble - set both to flat in both cars, then listen. Now crank up the bass level in each car and see what happens. Because of cabin dimensions, etc. it may take different settings to achieve the sound you want to hear. If you still hear no bass in the sedan, then troubleshoot for inop underseat sub-speakers, a bad amp (in left-rear of trunk behind access panels...), cut wire, etc...
2007 650i Stratus Gray Black/Beige, 6MT trans, ZSP Sport Pkg, ZCW Cold Weather Pkg, 609 Navigation, 610 HUD, 611 Night Vision, 322 Comfort Access, 677 Logic 7, 653 HD Radio, 655 Satellite Radio.