my .02
If you are going for "rear fill" don't use components there. buy a decent set of rear speakers and gain the correctly. rear fill is never ment to center stage the the listener so to speek. you need less power and output there. Some will disagree with me I am sure you have to decide what sound you like.
that being said some people will prefer the "center staged" sound where it is just loud front and back. you will ahve to judge that. you can pick up some infinity reference speaker for rear fill off
ebay for $60 or s then spend the rest on the front stage.
I personaly would rather have 150w and nice components up front then dencent speaker and 50w to rear fill. the thing that ***** about this is most amps are equal power amps. I would love a 5 channel amps that was 2x150, 2x50, 1x350@4ohm, 1x700@2ohm, 1x900@1ohm. on my last system I had a memphis belle and my fronts had 90w (could have possibly used a tad more) my rear had 75w had to gain them down. then the subs had 1100 watts and I blew up a set.
you will never go to a rock concert were you have equal audio front and rear.