K550 is quite neutral, so more top end extension than your HD 650. It’s biggest flaw however is the fit. If it doesn’t seal correctly and keep that seal you’ll experience very little low end response. So I highly recommend you try it before you buy! If it seals then it’s excellent, if it doesn’t seal it’s… quite lack luster
The Meze 99 Classic isn’t nearly as finicky and is another excellent option, I found it to have a bit more sparkle than something “dark” like the HD650 but it was also fairly warm & organic. Out of the options you have listed I imagine you’d really enjoy it!
The NAD P50 and ATH M50X didn’t impress me too too much, and I’ve not heard the HD 598C
Though what you may want to do is look out for a second hand ZMF T50RP I’ve heard a few of them and they have always been quite organic sounding. Do a little reading though as some are darker than others, and some are a bit more fun than others though all in all the ZMF House Sound is a fairly balanced take on a typical “dark” tuning
Finally, if you can find one and your willing to roll the dice the now discontinued Oppo PM3 is an excellent dark closed back! They run for around $400 or so and I would strongly recommend you purchase an Extra set of pads, so all in all you can likely get both for around $500!
Also whoa! I didn’t know Audio Technica had a new $200 option the 990Z looks pretty cool