best set up

I've never been really big on sharing an amp with more than one class of speaker.  Not for sound quality reasons, but for neatness and amp efficiency reasons, as well as options for equipment.  Having to choose one amp to power both subs and 4 speakers severely limits your options for all equipment as you are mainly limited by the power output on the amp.  Also, I have seen that situation wired all wrong, and poorly tuned to the point of sounding worse than the factory stuff it was replacing.  Multiple amps are to me at least, easier to tweak for their purposes, gives you a tremendously larger supply of equipment to choose from, and just in case anything were to break, it is easier to fix.  So in summary, if done right, you shouldn't hear any great noticable degradation in sound quality, but typically, 5 channel amps aren't powerful enough to drive a decent sub.  (unless you want just some filler bass)

I agree with silver, 5 channel amps dont have the push power and tend to be very large and bulky.  I'd get something like a 200-400 watt 4 channel amp, something small and compact enough to install like under a seat or the dash, to power the speakers and a mono amp (depending on what you want for bass) for the sub.

Hang on now...I have me a 5 channel soundstream and it sounds awsome! And it is not big and bulky thank you I have had different amps along the way 2 chann 4 chann and all in all the 5 channel amp is the best set up ive ever had. Everything is poportioned perfectly! And as far as sub power goes my amp gives out 150w@ 2 ohms which i have driving two 15s  and my mirrors shake quite violently