I probably shouldn't chime in as I've never heard the Denon. But I do love the sound of the HD650 for the kinds of jazz I listen to. Bill Evans, Jim Hall, Jimmy Giuffre, Bill Frisell, little bits of Brubeck or simlar and a lot of "New Acoustic" jazz-like substance with guitars and mandolins and such. Not so much big ensembles, more smaller combos.
What I feared about the HD650 is that my hissy old mono stuff (or remastered stereo) from c. 1960 would just be "revealed" as sounding awful. You'll hear advice hereabouts that better listening gear "requires" good source material or it's just terrible. Maybe the '650 is special or maybe that advice is well intentioned but wrong in this case. But with high-bitrate CD rips on an iPod, through a Schiit Asgard amp to the HD650 works very, very well on even the antique stuff with tape hiss, saturation, wow and flutter out the wazoo.
As a bonus, it works even better on well-recorded and mastered classical music. At least piano and chamber music which I listen to a fair bit of. For pop music, rock, etc. maybe the Denon will do better. I'm not disappointed with any of that but the HD650 is so darned polite and tightly controlled that you never get the kind of ragged, boomy sound that perhaps some artists would intend you to get on harder fare. Note that I'm not saying "lacks bass" but more like it lacks nastiness, even when a little bit nasty is probably the original intent of the track.
And if you happen to listen to jazz vocals...well, get outta here. Properly driven the HD650 will hit that out of the park like a hanging curve ball. The '650 doesn't do "forward" in the in-your-face way that some pop music is mastered but when you want the soloist to be supported by a warm backing from the guys in the band, now we're talking.
Heck, I even catch myself listening to that Kurt Elling "vocalese" album that my nephew gave me. On my old unamped HD595 it was pretty dull stuff, mostly. Really gets up and makes you notice on the new setup.
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