which hk receiver do you have?
what is your goal in upgrading?
hk receivers are not bad at all... unless you are wanting hdmi switching, I would suggest sticking w/ what you have and save for components, or something a real step up...
check out B&K components, and even their receivers if you want to look for an upgrade, but this is way above your $600 range...
just my opinion, but unless your hk is very very old, you won;t get much improvement in going with a $600 receiver...
then again, I am guessin you have a decent hk... if your hk is total junk, of course you could find something better...