Hi appologies for the delay in responding............ive lived with the quad s4 for a little while now, I personally rate these speakers very highly especially considering the price from Vickers Hifi. I ve listened to them on a Denon m40 mini system, audiolab 6000a & primare i22. The ribbon tweeters really are beautiful with female vocals, string & piano etc, there is something extra the uni Q tweeters on the kef r500s just don't quite deliver to my ears. Whilst they are relatively easy to drive, quad recommend an amp rated 40 _ 200 watts into 6 ohms they obviously deliver more with a more powerful amp. With my primare i22 rated at 80 watts they were sublime, yet still very accomplished & an easy listen with the audiolab 6000a. I tend to prefer less base heavy genres of music, listening to jazz, funk, electronica, classical, pop, country etc..............the one area these might lack for some listeners is base response, for me i liked the fact the base is always delivered in a tight controlled manner, never feeling over bloated. If you listen to predominently heavy rock these might not be the speakers for you, but then again you could always add a subwoofer? I haven't listened to the rega elix r, but would be interested to hear what they sound like together.
(Listening room 5m × 5m, with wooden flooring)