Yeah, I got both right next to me, I can try to describe what I'm hearing, but of course it's just one man's opinion and people might agree/disagree, as usual, there are many factors:
First I'd like to tell you that all my testing is done either on the NFB11.28 which I'd describe as a fast, neutral and analytical amp. As well as the portable Pioneer XDP300R. I do not own tube amps so all this talk is based on solid-state listening.
Also side note: I think, as most people here have noted, the original NightHawk pads work the best with NightOwls. Brings the best of both pads, good isolation and more neutral sound while keeping the bass and detail. But I'm ignoring that in this review and only talking about the two pads that AQ provides in the package.
Now, I feel like both headphones a have darker treble in general, but Owls' is darker by a bit, rolls off sooner. However there is a peak at around 3k and 10k that makes the treble a little energetic, especially with pleather Boost pads. Owls can surprisingly get a little sibilant with Boost pads but they are overall very easy to listen to. D7200's treble is brighter and very well extended in comparison and doesn't have no annoying peaks, but still a little darker compared to other headphones. The midrange on D7200 seems to be much smoother like its treble but it does definitely have that shouty nature (too forward to my taste) and can make the music sound less natural and more technical, Owls excel at making voices and instruments sound very organic and speaker-like, even if they are a closed headphone. Owls have a little lighter sub-bass over the 7200 but extend just as low, D7200 is definitely more for the people who prefer bass. The mid-bass punch seems to be about the same on both, depending on Owl's pads of course.
The soundstage and imaging definitely goes to Owls, it's just very holographic and has actual depth to the side and front
(after good burn-in and cleaning your ears 
), D7200 lacks in this regard and everything sounds very close to your face and intimate and while the imaging might be just as good, it's just ruined by the narrow soundstage imo
(shallow wood cups, thin pads, sealing issues).
Microdetail seems to be about the same on both, but you have to turn up the Owls to enjoy it properly, which is not a bad thing because they are very relaxed and don't hurt your ears. D7200 can get a little harsh when you turn them up.
Now one thing that people either hate or love: while Owls have an organic sound overall, there is a strange "muffle" effect present in both Audioquest's headphones, it's especially audible in vocals and it's as if you put a blanket over your ears, but more so on the Hawks because of its sucked out upper mids. Owls are pretty flat up until 2-3k. So right away, D7200 will definitely sound "cleaner" and more technical and you might think that Owls do not have enough detail and are just bad headphones, but Owls do take a bit of getting used to, just like Hawks. They don't shove all the detail and treble in your face like most other headphones we're used to, you have to listen for it appreciate it, but after a decent amount of comparing the two, I kind of think that both headphones have the same amount of detail and resolution, one just shoves it in your face much more than the other. Depending on your preferences, that might be a good or bad thing.
And I'd like to mention one more thing as well: I noticed that the AQ stock cable is complete garbage, not just because it's too short (seriously Audioquest, What?) but when I directly compare it with an upgraded silver cable, the stock cable just sounds very peaky in the treble and has a muffled sound overall while the silver cable offers a much more balanced treble and smoother midrange with cleaner instrument separation and less sibilent vocals, especially gets rid of that 3k peak. I never believed much in cables but this difference is very audible and I just can't ignore it. Add the OG Hawk pads into the mix and you've got yourself an amazing headphone. I'm using my Owls with silver cable every day of course but this comparison that I'm writing about is meant to be with stock cables as I don't expect anyone to throw additional 160$ for a cable upgrade based on my recommendation.
Anyway, NightOwls are a clear winner in comfort department. It just floats on your head and I can wear it for 8-10 hour without any issues. D7200 has a pretty stiff headband and it creates too much pressure on top of your head, the pads aren't that great either. They might seem plush and soft, but they always irritate my ears in some places and I constantly have to re-adjust the headphone, very disappointed here with Denons and it's the main reason why I'm selling them. I've read through the entire D7200 HeadFi thread and it seems that 70-80% of users seem to have some issue with D7200's comfort, so keep that in mind.
As far as sound leakage, since they are both closed headphones, they are roughly the same which means they don't leak much music into your surroundings, however it's very hard to get a proper seal with D7200 due to its strange oval pads and it may at times leak much more than NightOwl. Owls with Boost pads seal very well and leak almost nothing.
So in conclusion, if you want to know if it's a good idea to purchase the D7200 and you already own the Owls/Hawks and love them, then no, definitely not. The comfort on D7200 is just not good and they have their own issues with narrow soundstage and shouty midrange, very audible on female vocals. I'd go for something like ZMF Auteur straight away if you wanted a great complimentary headphone that is definitely different and technically "better".
If you already own Hawks and are enjoying them, it's worth trying the Owls too since you're already familiar with the AQ sound tuning and might prefer the more neutral tuning more.
I personally prefer the Owls - the darker signature along with some simple upgrades (pads, cable) make them a very worthy headphone for anyone who is looking to add a new unique headphone to their collection, especially for its current price.
I would probably not recommend it as an introduction to audiophile headphones though, there are cheaper and more reasonable alternatives.
Peace
EDIT: Pics or it didn't happen of course
