Snazzed, I want to give you some numbers from my system so you can put Ian's feedback into context.
I have an Onkyo TX-NR818. It's rated 135W/channel to a similar standard as your Denon. I recently did a test with a 200Hz tone via my M100v4. You know my room size and how far away I sit. My room is on the lively side so it's not too damped. As I mentioned above, I normally listen between -10 and -20.
At -20, my Onk is putting out 0.2W and at -10 it's 1.75W. I can turn my Onk up to +13 but for the purpose of this test, I turned it up only to +10 because I was worried I might blow something up. At +10, the Onk was putting out 173W to EACH M100. I doubt it was a clean 175W - I don't know because I had hearing protection on.
With that as background, let's say the sound is only clean up to +4 which is almost 45W. I think it's quite a conservative assumption to say it's clean up to a third of its rated specification. So even at -10, that leaves me with 14dB of headroom. In a 4200 cu ft. room and 13 feet away from the mains!
Your Denon is rated at 90W so let's say it's only clean to 30W. If I use my Onk/M100 data as a guide, when you set it to 0, that's leaving you with only 3dB of headroom. That would mean dynamic content is getting clipped. Perhaps you're not hearing harshness in the highs and instead perceiving that "it's not there" because Denon has done a good job on its soft clipping circuit.
Now that I've gone through this analysis, I can recommend you ought to give an ADA a try.
P.S. Make sure you don't have something silly like "Night mode" on.
