Sub and Components off 1 4 channel?

I have a pioneer 670mp head (22 RMS x4) and I'm running 2 polk db6500 components (10-100 RMS) in the front and the polk db650 2 ways speakers (8-60 RMS) in the rear.  What I WANT TO DO is buy an  Alpine MRP-F240 amp (40x4 RMS) and somehow run the front components off 2 of the 40 watt channels and then use the other two channels for 8 inch subs.  I'm not sure if you can bridge two channels while keeping the other two unbridged. 

Basically I want to power my components a little more while also powering at most 2 8 inch subs.  I'm looking at JL audio's 8w0 speakers (75 rms), Rockford Fosgate's P28S4 subs (50-200 rms), and Kicker's CompVR 05CVR84 (50-200 rms). 

This is my first car audio project and I'm looking for a nice tight and clean sound for my Montero Sport.  I don't need lots of volume or anything and I really don't want to have to get 2 amps.  If the above scenerio isn't possible power wise, will I be fine just getting a mono amp for the subs and leaving the main woofers running at 22 watts?  Thanks for any help you guys can provide. 

I haven't looked into 5 channels yet.  Most of them seem to be in the $500 range and I don't think I want to dive into that just yet.  Is it that important to have my rears powered and kicking as well?  My SUV places the speakers really low in the door and I can't tell how much they are doing now with both front and rear being powered equally.  Seems like it would just help out any passengers. 

Any one enjoy the sound quality of any of the above 8 inch subs?  Seem like they are still entry level but I need a low watt tight speaker.