Only you can tell what is inferrior or suppieror to your ears, but the 4600 has an improved amp section over the 2500, it is very likly but nt confirmed that the 2600 will use the same amp section as the 2500 but add extras.
The 4600 is a step up from the 2500 with respect to amps and the fact it has digital video switching, but the 2600 has supperior video switching cababilities. It has upconversion to digital and also video scaling upto 1080i i believe, i doubt neither would disapoint sonicaly, but if you take the 2600 to have the same sonic abilities as the 4600 that unit has been made redundant by there manufacturer in less time than pretty much any product in history.
I have a 2500 and it sounds great, i don't have any digital displays so have no need for the advanced features of the 2600, but when the 2700 roles around i would be that i'd have it (sould have a pj with digital input by then).
It will be better than your current amp, i moved from a ax620 to a 2500 and the surrond speaker came to life and prue direct has to be heard to be believed, i was also impressed how strong the radio tuner was, although the gui was a bit more hype than purposful for me (although the pretty graphs are nice) and the remote on my 620 was nicer IMO.