An amplified sub-woofer would be a great add-on if he doesn't have one already.
That Bass Link sounds good. Here's the important part of the review mentioned above if you have'nt read it already:
The sound quality from this subwoofer is amazing considering how small it is. The main reason for this is because it contains a 10 subwoofer and a 10 passive radiator. This allows the bass to be clear, crisp, and uninterrupted.
This subwoofer runs at 200 watts RMS, which isnt nearly as much as some of the subs on the market now, but it is definitely enough. My mirror generally shakes when Im driving with it, unless it is at a really low volume, and it is powerful to shake your seats, if that is what your looking for. If your not, then you can always turn it down with the remote volume control.
2004 E60 530is, Delivered 12/12/03, TiSilver/LGrey, PP, SP,PS, PDC, ACC, FRS, Step, Xenons, clear bra, body colored rear inserts. (Retired, 1983 henna red 320is)
2004 Corvette Z06 Commemorative Edition