One of the best pieces of equipment to add is a multi-band equalizer (the more bands, the better). That way, you can enhance the sound depending on the type of music. Deeper bass for rap, more balanced for other types of music.
The same goes for your home stereo.
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