Couple of other things
The specs of the 2600 and 4600 are very similar so you would expect the sound to be similar also. A couple of things I am aware of which are different between the 2;
The video up-scaling, up-conversion stuff (right to HDMI, 4600 is only to component)
Burr Brown DACS - meant to be good, don't know much about this
If you are running 5.1 (rather than 7.1) you are able to use the other 2 amps to bi-amp your front speakers
4600 has i.LINK (firewire) but only useful, obviously, if you have any players with firewire, which I don't currently
I will be interested to see what the 4600 is replaced with, I would have thought the 2600 was the replacement???? What will it be called? The 4700??