Nightowls are pretty dark. I haven't heard many dark headphones, but I really like the Nightowls. They have decent details, nice treble and bass extension, the mids don't sound recessed at all. After burn in; it's actually a thing, their timbre sounds less metallic and more natural. The Nightowls are probably quite polarizing because of Audioquest. I'm a cable agnostic my self, but they did many things right with the Nightowls.
I'll put it like this, I let my T1 go and kept the Nightowls. Were the Nightowl superior to the T1? They certainly had the bass I wish the T1 had. I got my T1 used, and honestly I was kind of bummed out by their build, I had the DT-1990 before and the T1 build felt lackluster, it looked way too similar to the DT-880. When it comes to detail retrieval and sound stage the T1 did have the upper hand, but it wasn't by an overwhelming amount. In fact, in some frequencies the Nightowls seemed to give more detail than the T1, it was mostly the low frequencies. The T1 were certainly brighter, no sibilance on my setup nor tiring, with a really nice timbre, it did the cymbals so well. The Nightowls after a while delivered a similar enough sound, that it didn't warrant to keep the T1 since I was using the Nightowl way more often because of their closed back nature.
Another decent dark headphone is the Sony Z7M2, I want to test it out in the local Sony store next to my Nightowl, but that will have to wait untill I get a balanced 4.4mm cable for them.