I have not read the review yet, but have owned the 650a and 650c for several weeks now. I have no hesitation in recommending both units. Why? For midrange clarity, low frequency control and high frequency expressiveness they excel. I'll explain.
My friend has had the 640a & c for a couple of years and although quite impressive, especially for the price, did not make me want to go out and buy.
I was recently out looking for a new TV and went into my local Richer Sounds, while there took a listen to the new 650 pairing.
Impressed me enough to buy, even in showroom conditions. Well they were playing a CD i'd worked on!
To give you a little background, my career is in the engineering side of the music industry, being involved in the studio with recording, mixing and mastering - so when I listen to certain CD's (mine) I like to think I know what they should sound like.
Over the years I have read many rave reviews, including equipment costing 000's, most never impressed.
Do most reviewers know what instruments are meant to sound like or for example a live kit of drums? And no I don' t mean going to a live concert and listening to it through a P.A. system. If you are close to a drum kit the bass drum pushes against your chest, a cymbal crash can hurt your ears! The pedals on a pipe organ have a hard to define breathiness. A live string section is breathtaking.
The 650's as a pair are able to portray the nuances of the instruments correctly, with a fluid well controlled bass and also have the power and dynamics in the treble to correctly define a drum kit, some would probably call it too bright but they are wrong - it is correct.
When paired with speakers capable of presenting the information they receive correctly, I have tried the 650's with several (including the studio monitors), they will not show themselves wanting.
Some of my colleagues (not in music) have systems costing £000's and will not admit that the CA's sound good. Pride?
Or maybe their ears are just used to the coloured sounds they believe to be correct.
I think they would go well with your RS6's but not sure how much of an improvement they'd be over the 640 V2's.
Edit: Seen the reviews now - just as expected!!! Maybe the reviewers ears are much younger than mine!!