Are you sure you cant buy the SHP9500? It would be my personal choice, and the price is kinda close to the ones you listed, well.. let`s go.
AKG K121 - This headphone was discontinued so it might be more expensive now that before, also the sound isn`t so good.
Shure SRH440 - Very good closed headphones, in fact I thinking about buying them later, the problems is that they really lack bass. But you can try to increase using equalizer but for metal and ASMR it won`t matter much. Fore pure music standpoint I would go with the Sure. As before tho, its all personal, maybe depending of your preference you will find the Shure too flat or boring. But in a analytical way to look these are the best in this list for sound quality.
Audio Technica Pro ATH-M30X - A good closed headphone but I prefer the Shure, they have more quality. M30X will have more bass and can be ``more fun`` to listen but overall quality of sound is better on the Shure and you dont need much bass for metal and ASMR. If you managed to get the M40X then it would be other story, then maybe I would suggest the M40X instead of the Sure.
Beyerdynamic Custom Street - Don`t know much about them to give opinion, but they never really got famous around this forum.
So it narrows down to Shure and SR850. As I said before, the Shure is closed and the SR850 is open. So you know the drill as I wrote it before. I think the sound quality of the Shure would be a little better, bass on both would be a little lacking. SR850 is better for Gaming and ASMR because of the open design also they also wouldn't be very behind in music, the SR850 are very good as well.
All in all the sound signature of both is kinda close, they have more mid, highs. The Shure would maybe be a little better for pure music in my opinion but would not be as fun for ASMR compared because they won`t be so airy and open like the SR850. Both headphones are a great choice but i`m inclined more to the Samson SR850 because the ASMR aspect.