OK I screwed up and order parts for my system and then ended up changing my design. So here goes:

Have (everything in RMS):
Kenwood KDC-X869 HU 27x4
Kenwood KAC-728S 300x1 or 100x2
PPI PCX-480 80x4 or 320x2
Infinity 63.5i front 75
Infinity 62.5i rear 75
2 x MTX Thunder6000 250

Ok so. I can either run the PPI in 80x4 and under power the subs a little at 300rms (instead of the 500 it wants) or I can run the fronts of the Kenwood 100x2, run the rears off the HU (27), and bridge the PPI to run 320x2 for the MTX subs.

I'm really leaning towards the 2nd option(98%), but my concern is will the rears be noticably underpowered? I know they are just for fill and really shouldnt need the full power otherwise they will overpower the imaging (or have to be biased towards the front more).

What are ya'lls thoughts?

 

I am wrestling with the very same issue and I am thinking I am going to power the fronts and rears equally with a 4 channel amp.
I have the same speakers and they really require deep power to sound just right.

I suggest powering all four off of a quality amp. You can always fade to the front if your imaging gets hammered.

 

If I power all four then my subs will take a hit from 320rms each to 150 each. Buying another amp now isn't an option. The question becomes will there be enough of an issue of underpowering the rear vs. underpowering the subs - which will be more noticable?

 

All cars will sound pretty different as far as necessary relative power to each respective set of speakers. But to make a generalization I can say this: Overpowering your subs is as bad as underpowering any other speaker(s). It won't hurt to try wiring both ways. If you are really into bass, you might like the rears off the HU better. Knowing my personal taste, I would definitely run all off the amp.

 

run the rears off of the HU, and turn them way down...your system will sound much better without the rears there at all...they are only there for rear fill, and since they are right behind your head, they need to be turnbed way down anyways...

Dedicate your power to a nice set of front seperates, and be sure to give youir sub plenty of power.

Also, don't worry about rms ratings of speakers...jsut be sure that you provide nice clean power, and that you use good wiring to the battery and to your head unit. A 200 wrms sub will handle 1000wrms easily if the power is clean...A 500wrms sub does not require 500wrms, and does not require that power, its jsut a stupid rating that, to me, doesn't have that much significance...

 

Thanks for the help guys. I think I'm going to go ahead with with you said onecoolee. I think my install will start next weekend, so I'll post some pics of it once its done.