Unfortunately the best way to decide on a receiver is to listen to it which, as you stated, is sometimes not an option. That's one of the reasons I tend to stick with things that I can find locally )Denon, Marantz, Yamaha, Pioneer Elite). For me this does include Aperion speakers since I live about 60 miles south of their headquarters. However, even if I lived on the other side of the country I'd still consider buying them since they have the nice return policy.
Of course, some of us live in places where no local dealers exist which means we have to narrow our choices down based on specs....
No to mention the fact that the dealer thats sells X receiver may not sell Y speakers, etc.
Anyway, based on the listed specs (of the 635 since I can't find the 645) it should be a good receiver. The power seems a little low, but if I recall correctly, HK rates their power output differently than most other companies. I think they rate it by actual full-range output which is probably more accurate.
I still recommend finding a dealer (if possible) that can demo each of these receivers for you. Even if you can't demo them with Aperion speakers you can at least get an idea of how each will perform.