Considering adding an amp

[Indyjones]

Well, I'm finally considering adding an an external amp to the system, with the prime canidate being a Emotiva XPA-5. My current setup consists of this (only running it at 5.1):

receiver - Pioneer Elite SC-05
center - Polk Audio CS400
mains - Polk Audio RTi 8's
surrounds - Polk Audio RT5's (yes, they're old!!!)
sub - pair of Definitive Technology 15TL+

One day when I hit the lottery, I'd like to get better speakers, but for now, these will have to do. I have a few questions, yes, probably dumb, but I am a newbie at the seperates side of HT.

First, would you add an amp to this system? I feel the way the system is currently, it does a decent job, but there's always room for improvement, at least with my gear. One concern is my current room is roughly 14x12, which isn't very big, so I don't know if adding an amp would be overkill. Also, how does the sound quality compare between an AVR and amp, besides the more raw power output? Is it noticeable or margainal?