Hi Bruno,
Much good advice already given, but I'll address a couple of your questions. Your Denon gets very hot when you are driving it to very loud levels because the output transistors are driving 4-ohm loads, the M80s, so more current flows through the output stage and it heats up (power is measured by both voltage and current).
The Denons do not seem to have thermal sensors on the output stage, which is why they will drive the M80s whereas some other brands historically will not. Older Onkyos used to shut down very quickly when driving M80s, either because of overly jittery protection circuitry that sensed too much current flow or because the output stage got hot. Newer Onkyos are better.
Consider "0 dB" on your volume readout as a rough guideline that you should not exceed, or if you do, you may risk "clipping" the output stage or incurring increasing distortion that may become audible.
Manufacturers still get away with a lot. In the old 2-channel days, most all receivers drove 4-ohm loads without difficulty.
In current av receivers, there are 7 internal amplifiers, not two, and dissipating heat is a big issue, so many manufacturers will recommend you do not connect 4-ohm speakers, or they set the protection circuitry to shut it down, on include an impedance switch that on the 4-ohm setting severely limits the power/current flow through the output stage (much reduced power output).
As to subjective volume sound levels measured at your seat, "85 dB" is interpreted by most listeners as "quite loud"; "95" dB as "very loud" or "twice as loud" as 85 dB; and over 100 dB, extremely loud. By the way, the levels I've measured of orchestra and chorus in New York at Carnegie Hall or various opera houses, never exceed peaks of 105 dB SPL (sound pressure level).
Rock concerts can be absurdly loud--dangerously so--and may approach 122 dB, not far from the threshold of pain.
Oh, ignore specs of "Dynamic Output"; this is totally bogus. It's quoted to get the numbers up. No regard to distortion, always a single frequency. Useless.
The correct spec is Dynamic Headroom and is quoted in dB, which most consumers do not understand, so it's never quoted.
Regards,
Alan
