I might be able to help you decide, I just put the 2 ohm Infinity 9622i'* in my monsoon SSEI.
After my wife drove it for a week and broke them in, I am VERY happy from the bass, a little bit suprised, someone on here described it as near sub levels.
I can rattle the plastic panels in the car with the rears, it doesn't rattle the license plate like the 2 rockford 10'* I have in my aurora however.
If your looking for more bass than the 2 Infinity'* put out I suggest the infinity basslink for a system that doesn't eat real estate. I had one in my Grand Am and it put out just enough to rattle the license plate.
BTW the rears are filtered but thats not why they sound crappy, I can't figure out why for only 100 retail I can put in such good speakers to make great bass and GM can't do it wholesale.
The 9622i'* are a 2 way system meaning they are intended to also put out treble and mids, while you won't get any from them and the tweeters in them are a waste in our case, they sound great and they cause no problems being in the car.
A perfect sollution would be a 6x9 woofer but I haven't seen an aftermarket one for 18 years.
If you really want to go all out and have near stock looks, custom install 2 8" subs in the stock locations with amp.
Whatever you do, DO NOT GET 4 ohm speakers, you'll be ticked with the lack of bass.