Hi there.This is what I use the Nio for as well. It has the visceral bass you're talking about. For faster metal it can feel a little sloppy. Another great option is the Thummim. Chunky guitar riffs combined with treble extension and a holographic soundstage. Some have described the Thummim as the TC in IEM form. I find it difficult to compare across IEMs and HPs, as I don't think it's possible to do what the TC does in IEMs, but I can see the resemblance I suppose.
I see that you have and / or have tried almost all the best in IEMs, and that you also enjoy metal.
My preferred IEM, so far, is the Mest. I do not feel that it lacks bass almost at any time (IMHO). I also really like the other 2 IEMs that I have right now, the Clairvoyance and the Isabellae.
However, lately I have noticed some discomfort and sometimes even pain in the ears, after using the IEMs for a while. This has worsened after a tinnitus appeared in my left ear this weekend (I'm much better today).
My impression is that my ears are bothered by the air pressure when I am using IEMs. This has never happened to me when using the U12t, but I sold it because its sound was not to my total liking (I was missing a bass DD, basically).
For all this, I was thinking maybe of selling the Clairvoyance and the Isabellae (I will keep the Mest, because I love it), and look for a 64a with DD driver that I might like. Among the options (Nio, Trio, Fourte) I think I would put the Trio first, without having tried any of them. I think the Nio would lack treble extension for my liking, and the Fourte is out of price for me.
How about you see the Trio for rock, progressive and metal? How would you compare it to Mest? Is it worth saving and paying the difference in price between the Fourte and the Trio?
Thank you so much for everything.