IMHO, the most audible difference will be the room correction/subwoofer integration via Audyssey (particularly xt32 on the higher end models) on the newer models. IME it will swamp any performance delta in the DACs themselves - particularly if (as I assume) you are using subwoofers. Some like to go the other way on this issue, but I'm a true believer.
Marty
Marty