I've owned the LS50s for almost 4 years now, they work well in my room. From my limited personal experience, and what I have read on the net, they are finicky when it comes to amps. Not that the Nuprime is not worthy, just not a good match. The first amp I used to drive my KEFs was an Aragon 8008 model, decent amp but didn't work that well either. Plenty of needed weight on the bottom, but the highs were irritating. It's the upper midrange/ lower treble of the KEFs that can be a problem.
The LS50s seem to be rather controversial. They got that "class A" recommendation, but many folks didn't like them when they heard them, for what ever reasons, and made it a point to proclaim that to the world. Stick it in the eye to Stereophile, if you will.
I think many owners who did like and buy them are now trading them for the newer powered model, thus the ones on the used market.
I bought mine cause of the decent reviews and more importantly I could get them through my Amazon Store card (free financing). Same way I bought my Nuforce MCP-18, which is used as the preamp to the KEFs.