Let me see, how to quantify my answer. I think Q40 rumbles a bit more and may have a touch more mid bass but that's really splitting hairs. The sound the Denon has is way more musical (not sure if that is a proper description) but has clearly better mids and highs.
Kind of the all around I've been hoping for. If they continue to please me like this I will likely never ever sell them.
Remember too, these have the angle pads and MarkL mods to them so they are a cut above a stock D2000, which I have never heard.
I felt the same way about the Denon D2000's. Although they are at a much higher price point, the D2000's were clearly the better headphone. Better mids, brighter highs, bigger soundstage, better comfort, and build quality. To me, Q40's felt and sounded like $100 headphones. Very good bass but the mid range, soundstage, and comfort weren't enough for me. I returned them after a week of usage. This is all IMO.