Why were you thinking it's broken? Percentage is a poor way to judge volume as you don't have a reference as a percentage of anything except maybe the max position of the knob. Most avrs use dB based volume controls referenced to the THX standard....Sony didn't do this for some reason. What actual readings on the dial might be more helpful (they're not percentages, they're still likely dB based, just hard to know the range) . Motorized control knobs can look kind of odd using the remote.
4500 for that price spread for me, slightly better amp section in any case altho a slightly older design....I'd be hard put to know if there's any tech feature/advantage that you might want that might be in the 3600, tho. Denon site has a comparison tool....
AVRs and even many power amps have a single power supply, so the more demand from more channels the less there is to spread around so to speak, but the specifications should tell you the details of its limits. All channels driven is not generally a real world situation for music/movies either, tho....altho folk who like all-channel stereo settings loud for music, particularly with full range speakers and no sub. For surround movies/music the surrounds just have less needs. The front three, particularly the center, is where the power demands will be.
Somewhat depends on spl levels you need for the speakers you get at the distances you listen from....try this spl calculator
http://myhometheater.homestead.com/splcalculator.html to get an idea of the relationship. OTOH for just 2ch stereo that power supply dedicated to just 2 channels will yield it's highest output. You can use bench tests of avrs to get an idea of power at (typically) 4 or 8 ohm impedance, number of channels driven, clipping, etc....
