Well, I guess it comes down to budget?
Morel are always my faves, but they are not cheap.
DLS always do some fairly good ones at the right price.
For mid bass rears I would guess something like the Earthquake SWS range maybe?
Focal used to make a 6" mid woofer that was great, but no longer so used only.
I think with some well mounted Morels up front, a half decent amp and some mid bass drivers in the rear you would be away.
But what is your budget?
The Alpine MRX-F30 is a pretty good amp, and I am not an Alpine fan, but great value for the sound quality, not as bright as many Alpines.
RRP is around £199, expect to pay around £180.
You could add the amp first and see what you think, it will improve things 10 fold just driving it properly.
I actually think the speakers in the w211 are pretty decent, and unless you mount new ones really well you wont see an advantage.
So do the amp first maybe?