Bruno, I just did a quick search for lab test results on the 890 and didn't find any, but I did find
these on the Denon 2309(889), which should be close enough. Generally on music distortion has to be above about 1% to be audible, and the Denon unit is capable of driving both 8 ohm and 4 ohm loads cleanly to beyond safe listening levels. It appears that what you describe is unlikely to be related to amplification.
It's good to have an SPL meter for determining the sound levels that are actually present during listening even if(as I do)the calibration of levels is done by the receiver auto-calibration system. We should always be cautious about sound levels which cause hearing loss. This also allows more accurately judging actual power requirements at various moments.
Your situation may be due at least in part to room effects, so experiment. It may be difficult for you(and impossible for us, of course)to come up with the best solution.
