I'll tell you my experience.
I have always been on the fence about adding an amp. And thats generous. I used to think power is power. A watt is a watt regardless of where it came from. I think I bought into the idea that unless I double the wattage of my AVR, I wont hear a noticeable difference.
Then I got an XPA-2.
Wow. Was I wrong. I understand this is an endless debate. But for me, at moderate and higher listening levels, I heard an IMMEDIATE difference. I mean, the overhead opened up the mids...on some tracks I heard not only the fingers sliding across the frets I'd never heard before, but heard the PICK on the strings of the guitar. And these are tracks I have heard probably 100 times before.
That said, addressing another issue, first I ran MCACC with the amp active. I then gain matched my subs, and finally level matched my 5.1 system with an SPL meter...all speakers to 80db. Even in 5.1 scenarios, at no time do my mains I have amped sound overpowering or stand out. Everything blends. But when listening to 2 channel music it simply sounds amazing.
Just my experience.
Pioneer Elite VSX52 AVR
Polk RTi10 fronts (powered by the Emo)
Polk CS3 center (powered by the AVR)
Polk FXi A6 surrounds (powered by the AVR)
My system: Yamaha RX-A3060, Emotiva XPA-2, Salk Song3, Salk Songcenter RT, Polk FXiA6, Rythmik FV15HP, Intel i7 HTPC running JRiver Media Center