Picked up an inexpensive pair of MB Quart 6x9's.
With the advice of one of the posters here I popped open the side panels (after removing the seat bottom) and went at it.
Some of the instructions say to pop the seat upwards after removing the plastic inserts for the baby straps and just pull. I don't know which car this was for but on my 06 you need to push the back of the seat down a bit to unlatch it and then pull out. There's a small "hook" on the back that holds the seat from moving forward.
I also recommend having a few of the mushroom plastic connectors handy because mine were so fragile that if they didn't go in perfectly they pretty much just cracked and fell apart.
Otherwise, job was quick and easy.
The speakers made a HUGE difference. WAY more bass and I can hear crystal clear high's!!!
My system is just the regular one but I can't believe the difference in sound.
Probably the most inexpensive and satisfying upgrade I've made so far to the car!

 

Which model of MB's did you use? Two way's, three ways?

 

Just two way coaxial speakers.
I think these were the Discus line.

 

I put in a set of Polk 6x9's in the rear of my R53. Made a huge difference. The input is off of the front speakers, so there is no missing bass.

MB Quartz components up front
Alpine Type R sub in the back
Alpine V12 5 ch amp pushing it all

 

Might just be me but I thought I had tighter and more bass with these speakers.

 

Where do u have ur amp mounted? Any pics?

 

I had an Alpine KTP445U Power Pack with my OEM non-HK installed today. The left F/R speakers were connected in parallel (final = 2 Ohms) as Channel 1 and connected my 2 Polk Audio Bookshelf (8 Ohms) home speakers as Channel 2 in the cargo area. The rear channel of the OEM stereo can produce very good bass with the right speakers and better amp contrary to claims that it cuts off after 100 Hz. It's the OEM rear speakers that sucks. The OP has a point.

 

No pics handy but the amp is under the passenger seat. If you take up the carpet there is a styrofoam piece about the size of an amp in there, provided you don't have nav.