Hi Bruno,
Agreed, extremely unlikely that it's room related. What you might be hearing is "incipient" clipping. That's when the amps in the Denon aren't actually chopping off the tops of the waveform, but getting close, so distortion has risen to audible levels.
Keep in mind that all amplifiers driving 4-ohm loads will have higher measurable (not audible) distortion because of the increased current flow through the output transistors---more heat, more distortion. If you look at the distortion curve on an amplifier as the power output increases, you'll see the distortion figures ramp up just before the amplifier reaches the "knee" of the distortion curve when it actually clips.
I suspect that you were pushing the Denon into this territory that precedes actual clipping, when audio may become somewhat edgy, mushy and unpleasant. I've heard similar qualities from both tube amplifiers (the stereo soundstage collapsed into mono and the sound became mushy) and transistor amps when pushed to levels at almost-clipping.
There is also a psycho-acoustic effect wherein your ears may have reached their limit in terms of loudness tolerance. When that happens (to me and to others I know), the sound subjectively is intolerable and may simply sound "distorted" even when technically it's clean.
Regards,
Alan
