The 3600 would not be a worthwhile upgrade over the 2600, as you suspected. The power difference is marginal and feature set/connectivity nearly identical.
The current top of the line HK (3650) only adds DPLIIz and HDMI ARC over the 7550, despite it being a few years old. The 7550 is substantially more powerful, and has several other features that have only been seen on the top-end HKs (e.g. EZsetEQII, dual independent subs, Logic7 layering on multichannel streams, bass management on multichannel analog input, etc.)
In this post I made a detailed comparison between the AVR 7550HD, 3600, and the current 3650 features;
I am a big H/K fan, but If you want any 'newer' features than this which have become commonplace on most other AVR brand lineups, (such as airplay, AVR control apps for your iPhone, pandora, dual HDMI outputs, etc.) look into brands other than HK. Other brands typically offer a better bang for buck and roll out new models+features much faster.
New HK models aren't due until next year and it's uncertain what new technologies will be included.Edited by dRockHK - 8/5/12 at 2:55pm