The RXV2600 is the only one of its kind on the market. There are three main processing modes pertinent to this discussion; deinterlacing, upconverting and upscaling.
Deinterlacing - Progressive scan, same thing. Takes a 480i signal and processes it into a 480p output.
Upconverting - Processes an incoming 480i/480p signal so that it can be transferred out along a digital cable (HDMI/DVI).
Upscaling - Processes an incoming signal to a high-definition output. Not just making it HDMI/DVI compatable, but actually improving the signal to a HD format (720p/1080i).
The RXV2600 is the only Rcvr on the market which upscales. Also, it has HDMI switching. ** Note ** It does no processing of HDMI inputs, it merely passes through the signal. So if you are not using an upscaling DVD player, you will WANT to use the Component video cables. That way, your RXV2600 will detect and process the 480p signal to 720p/1080i.
As for sound quality improvement, it is the DACs! The 2600 uses PCM 1791 Burr-Brown DACs for each channel, an significant improvement over the 2500 processing.
The Denon is more expensive by far ($900 more) and does not upSCALE. It does upconvert and uses the same DACs.
The Pioneer Elite VSX-72TXV is $100 cheaper and does not upSCALE. It only upconverts 480p and uses the same DACs. It does have the added bonus of an IPOD connection, if that means anything to anyone.