Well, with those of us with the 97 -2003 528i lower end radio, I think we're all in agreement that the sound... well... sucks. I added Boston Acoustic RC520 5 1/4 Rolly Comp speakers in the front and traded out one tweeter (both doors) and 5 1/4 2 way Neo NX57 Boston Acoustic's in the back. What a difference! I didn't boost the amps, I didn't swap out the head unit and I didn't add a sub woofer, but still an incredible new sound world inside my car.
For those not happy with your sound, swap out the speakers first with quality new ones and you'll be happy.
If I add a sub woofer, I think I'll add a tube jobbie that is velcro'd in and hooked up so that it can be taken out if I need the additional trunk room on a long trip. I'm an older fart so I don't need the car to sound like a travelling bass drum.
But if you're not happy with your 528 sound, swap out the speakers first.