Ok, so first of all: I really admire your take on this challenge, and technical challenges in general.
You always take the opportunity to really think about your icon-making process and to challenge yourself to both improve upon your style and to get out of your comfort zone.
I really like how meta you go about this kind of challenges and it's always interesting to read about your thoughts while planning your sets.

Your axes are great, I especially like the gritty/matte to smooth/shiny one. It was a clever choice to make it the z axis because imho it's easier to gradually go from bright to dark or from low contrast to high contrast, while it's harder to go from gritty/matte to smooth/shiny in more than 2 steps.

I'm going to repeat myself, but your mean color square method is a thing of beauty and you're brilliant. Congrats on your brain, as usual.

AND NOW, THE ICONS THEMSELVES.
Let me wax poetics about 8. Please let me. Because it's a masterpiece. I love the low contrast, I love the darkness, I love the background and I absolutely adore the subtle text. The font choice is spot on and it makes the icon look like a poster. The kind of icon that makes you wish you had made it.

Other favs (they're mostly from the matte/gritty square, but that's just because I'm very partial to matte/gritty icons): 3 (gorgeous cold coloring and interesting texture use), 4 (the texture work is exquisite and I envy the lovely green coloring, plus the text is perfection), 6 (oh the stark contrast between the peach background and the perfectly extracted black silhouette! oh the font choice!), 7 (very dramatic!), 10 (very beautiful lighting and very interesting composition!).