Thanks for all the replies
I had suspected that the difference was mainly efficiency. I probably wouldn't be able to tell the difference between a good receiver with class AB amp and a good receiver with class D amp.
I think I'll just keep my 2nd HK-3490 (on the way) and split the speaker wire right this time. I was too lazy to do it right the first time. However, I'm sure it sounded like shit because I didn't split the speaker wires right (I put it in the binding posts with a lot of plastic still wrapped around the copper wire so the current couldn't pass through very well). The copper wire got really dark after my first week of use so I used headphones since then. It finally blew when I tried to connect some computer speakers to it when the power supply was switched on. Fortunately, I got the sage max warranty when I bought it.
I'd like some 3 way speakers but don't really have the money for them. If the amp weakened, then is it likely the speakers (a pair of infinity Primus P162s) weakened also? If so, then I'll have to use headphones at first until I have money for some good speakers.