Hi, Firstly I am not in a position to give advice. However, here is my view.
I used to own a 540Rv2. Reason I bought this was cost, size and good reviews. I ran this amp with M-Short Genie's (6.1), reason again cost, size and looks. I was thrilled with this.
For no other reason than wanting Inncognito, HDMI switching and 7.1 I bought the 640R. Again, I am thrilled with this. I have 2 Sky HD boxes, PS3 and DVD89 (Tv only has 2 HDMI sockets).
Yes I think the 640R sounds better (I do like to listed to sound at low volume) so the high power is great. However, based upon sound quality only, the 640R is not £330's worth better.
So my point is, if you don't want incognito or you don't need 7.1 then don't get the 640R and look at the 540Rv3
If you dont need HDMi switching then look at the 540Rv2
If you don't need 6.1 then get look at the 340R
I don't think anybody makes and better sounding Amp than CA at their price points. Yes you will get more features, but at a loss of sound quality.
Andy - Do you use the 7.1 or the HDMI Switching or the inn-cognito. Then is so, you have getting your full value over the 640r which you would not have got over the 540rv2. That inncogniote via 640r is a fantastic way to get multi room audio and multi room video control (I don't use the composite video feeds prefering to go direct with HDMI but, I get full video control via the 640r - this in itself would cost a fortune otherwise)
Mark