Well, I did the same as you. Stage one Kit, Ghost Subs. I have an E92 that had the HiFi sound system. At first just had subs, improvement. Then got Stage one kit. More of an improvement. Then got Bimmertech Amp. HUGE improvement.
Now granted that alot of you are looking for Bass kicking sound. You won't get that with this set up as you don't really have a Sub Woofer. You have 8" ghost subs that produce more of a punchy mid bass. Snare drums, kick drums sound great.
Just recently added a subwoofer box and amp. My system is now complete. It pumps and thumps. Got a Bassworx sealed box. Hertz 250D 10" sub (dual voice coil) Hertz HCP 1D amp.
As I have said before in other threads. Bavsound speakers may be a little more expensive for what you get. But to be plug and play and install for the speakers took no longer than about 2 hours for doors and floor subs. It's a pretty great sounding system. Now with the addition of a sub that goes fairly low, I'm happy.
BTW, I used to work in a car audio shop back in the late 80's early 90's and had a friends car that took a few trophies in sound off's back then. Even had his car in Car Audio magazine.
Any questions, fire away.