I had a short (one hour) comparison between the Nightowl+leather pads, my modded TH900, stock Denon D7200, Oppo PM3, AKG K550 Mk2, and a bunch of others (Hifiman HE-1000, 560, Susvara, Oppo PM-1, HD650, HD800, B&W something, ...). I know one hours is not much, but I got pretty experienced on what to look for and what music to test with.
The Nightowl delivers what it promises - not much treble, but you get used with the sound. Not the most bass extension, but good enough. I would prefer a bit more open sound and bigger sound stage, so I guess that means either other pads (suave or hybrid). I guess this is possible with pad changes.
The Denon D7200 (I hate when I have to write Denon, but otherwise I'd associate to Nikon from the D7200
) is the most similar to the Nightowl. They are like equal competitors in the treble-trouble-free zone. There are no factory tunings for the Denon (like pads for the Nightowl), but I know from experience the D7200 will change a lot with different pads.
The Oppo PM3 was the most balanced closed headphone of the bunch. A safe buy for everyone. I can image a pad change could also make a bigger sound stage and deeper bass.
It was interesting how close these headphones come to yesteryear references like HD650 and HD800, even besting them on some terms.
All Hifimans had the usual Hifiman problems I have with them, mostly harder mids (evident on piano recordings) and a bit thin sound (depending on model). I would choose the PM1 + some mods over them.
And now we come to the star of the show, the K550 II. On instrumental music it sounded more linear than the TH900, with similar stage and depth, totally satisfying. What a surprise. Less deep bass though, and needs good amplification to sound good. Also, outside classical and instrumental (e.g. pop, EDM, heavy metal, rock) it didn't sound nearly as good as the TH900. However, I rate the K550 II very high, competing up to 10x its price range with instrumental, classical, and contemporary pop music.
For my taste, the modded TH900 pretty clearly outclassed by one or two levels the above closed headphones with most musical genres, except instrumental/classical on the K550 II. It was an "ahh" experience every times I switched to them, they sounded smoother, more contrasty, greater dynamic range and better scale - but more colored too. And it's still my #1 bass-rattler headphone. So far I didn't have luck finding a TH900 replacement, but got surprised by the K550 Mk2. Next to try is the K872. The D7200 is on the keeper list too, though fixing its midrange issues is still on my backlog.
So if someone is looking for closed cans and sound is the priority, today I'd recommend the TH900, Sony Z1R, (perhaps the K872), then the K550 II, then either the D7200, PM3, or Nightowl (these all need mods, then it depends on the results). Even in stock mode, they are definitely competitive with open back headphones in most regards and most musical genres, though differences apply.
If comfort+sound is priority, the order is Z1R, Nightowl, TH900, K550 II. The PM3 and D7200 may be a bit small for certain people, they were on borderline with me. I would use somewhat bigger pad opening on the PM3, maybe it would become a bummer.
If price+sound is priority, go with the K550 II and an amp (e.g. at least Furutech AGL or better). I am still in awe.