Well, let's start with determining differences in sound quality between two amps irrespective of price. Audioholics.com, which I consider to be the most objective (perhaps at times bordering on simply being close-minded) site on audio, certainly has left the door open to the argument that people *can* hear differences:
It also provides a good primer and discussion on what technical details of an amplifier's design influence sound quality and whether (or which) measurements correlate to perceived sound quality.
Personally, I've heard differences in amps - I've brought in my integrated for a head-to-head comparison against an Ayre AX-5 and I could definitely tell a difference. Now is that difference going to translate to my system in my room? I don't know.
I also don't know if it's worth the $10k MSRP for the AX-5 - but that's a different question entirely.
Now for scaling amplifier power, let's look at a handy calculator:
The dynamic range of a compact disc is about 93 dB. If you sit 3 m away from the speakers and want the amp to reproduce the full dynamic range of a CD with the R300's without clipping, then the amp would have to output about 57 W. Let's say that KEF was a little optimistic about their sensitivity rating for the speaker and it's actually 85 dB. Then you'll need power output of ~113 W. If most of your listening is done at moderate volume, say around 75 dB SPL, then you can get by on a measly 2 W.