I've been letting the speakers cook with daily usage from a vintage Dual
CS5000, Schiit Mani, and Ortofon OM20 combination. At moderate levels
the difference between the old *Wharfedale Denton and the KEF Q350
speakers are subtle. The Q350s are a touch cleaner with a brighter
tone. Detail - neither are what I would call overachievers here - is
also a little better, perhaps the difference between aluminum and woven
Kevlar drivers. The bass out of the 6.5" KEF woofers go down a touch deeper
and with a bit more definition than the 5" unit on the Dentons.
With the family out of the way for an hour, I fired up the Thorens TD309 and spun my US copy of Pink Floyd's The Wall. At a higher listening level: within their dynamic limitations, the KEFs do a bit better here too. The busy sound effects come across cleanly and have real depth too. Definition - inner detail and body - are certainly not the best I've heard. These are, after all, no electrostatic speaker or an Altec 604 driver with a time-aligned crossover, but given the price the KEFs are more than acceptable. The old "PRAT" factor is high with lots of toe tapping enjoyment. Bass with the 6.5" driver is good but if you want to go deeper and louder a sub (or a larger Q-series) would be needed.
Further listening with a variety of records revealed a clean midrange with only some minor veiling - again, compared to the best I've heard. ie - there is an ever-so slight blurring of aggressive guitar work and the leading edges of high-hats. The treble is, however, extended with shimmer and shine but still not etched. I was expecting some overdone brightness from the aluminum tweeter but my (aged) ears weren't driven out of the room.
Overall I would give the KEF Q50s a solid thumbs up. They do punch quite above their cost point but these days that's no big surprise given the available computer modeling and testing that would have been a dream for speaker designers in the past.. I'm curious how the R Series would perform. But that's a future hope.
*Regarding the Wharfedale Denton speakers: these are equally good speakers but something I would recommend with a more aggressive digital front-end or amplification chain. In my case the smoothness of the Aleph J and the slight darkness of the Classe Five preamplifier was perhaps too much of a good thing. The Denton appears to have been voiced to have a vintage, smooth sound instead of the "hi-fi" approach of the KEFs.
System:
Classe Five preamplifer
Aleph J amplifier
Thorens TD309 turntable with Ortofon Bronze phono cartridge
Belden 9497 speaker cable
various budget interconnect cables 
