Couple months ago I'd also looked at the Onkyo 8511, HK 3480, and the Denon 685. The Onkyo 8511 runs much hotter than the HK and Denon, I can not imagine how hot would it get if I were to run it at 4ohm so I didn't feel comfortable getting this unit and scratch it off of my list. The Denon didn't sound too natural to me and it was a bit on the bright side, cosmetically it feels cheaper than both Onkyo and HK, the Denon didn't play as loud and it was clipping at about 80% to max while the HK and Onkyo still doing comfortably at the same level, though the HK 3480 has lots of headroom left, provide the cleanest and most natural sound, and only slightly warm to touch after hours of cranking. Personally I also prefer the HK appearance eventhough it was not my purchase factor, so it was a no-brainer to me and I chosed HK 3480.
For your M80, some people tend to think it's a little bit brighter than the M60, M22tii and M2i, plus you need a receiver capable of 4ohm to run them, so of the two you'd mentioned the HK 3480 will smoke the Denon all day long, I do not think the Denon is capable of running at 4ohm, even if it does...I bet it's going to run so hot you can BBQ with it.
I'm sure there's other option out there, including the NAD 320 and some Rotel that you can consider, though they cost twice as much and only put out about half of the power of 3480. Sure, you can look at used stuff, but it's a 50/50 chance that you might get screwed over, personally I don't buy used stuff and I hate auction, unless I know the seller personally otherwise I prefer to get my stuff brand new. The HK 3480 only going for $299.00 with Free Shipping from J&R, so I don't think you can't get any better deal than that. As far as I'm concern The HK3480 is the one and only stereo receiver that have the Sub pre-out too!!
Another option to consider is to use your current Technics as a preamp, and get two Outlaw M 200 monoblocks, these bad boys are putting out 300 watts @ 4 ohms without breaking a sweat and it will drive your M80 to reference level. I think they're running for about $500 for a pair, I'll definitely go this route when I get a pair of M60 in the near future.