Well the Onkyo runs hot because of insufficient heat sinking and the HDMI boards in Onk's generally run very hot anyways for whatever reason. As to why they both sound equally good, if both amps are running well within their capabilities there's no reason why they shouldn't sound pretty similar.
I have an SMSL amp and have had it for about...........hmm must be about 3+ years at this point and it's always been a great amp for me. I've used it to drive just about everything I own at one point or another and it does pretty well for a little guy.