All of the KEFs I've ever experienced benefited from a high current amplifier. In your case, I would compare that spec. between the two amps and purchase the winner...
I looked up the spec on the R500 speaker, min 3.2 ohm, recommended amplifier power 25w to 150w. Any reason you want a 250w into 8 ohms amp for this speaker? I'd be thinking more towards an integrated from Naim, Exposure, Croft, etc.
For the record, I have not heard the R500, just offering something to consider.
You don't actually use all the power an amp can produce in excess of what your speakers can handle according to the manufacturer. So having too much power, is like having a 450 HP engine in your car, knowing the speed limits in your area would have you barely touching all the juice the car has. When it's needed however, it is wonderful to have. I think the Kefs may be a current hungry design regardless of their wattage recommendations, the minimal impedance is important. They weren't current or power hungry in their early designs, esp. examples that Nud Fried helped design, years ago.
The real problem comes down to what kind of sound do you want. The W4S is class D I think . I have never loved any product so far with that class of power. I have heard Naim and if your budget will accept their amps I would consider them very seriously, but you should really try to audition them yourself. Did you mention a budget?
I know the speakers. More power will help to improve dynamics, bass response and control. I decided on a Musical Fidelity M6i.
They are ridiculously good speakers for their price. Truly a mega-bang for buck product. U should find a pair to hear them!
I think a well chosen $3k retail integrated is reasonable for the R500's. I like MF products.
I am sure Naim and the others are great as well.