Goomba, now that I see it all together, I have to admit I don't love the plywood. It sort of has me asking the question, "why?" Why use a different material on the landing than on the treads? If you're going to do that, I'd think you'd have a purpose for it, to make a statement of some sort, but then you're trying to stain it all to look the same. I dunno, it just seems like you're caught in between using a different material to create a contrast and trying to hide it by staining and finishing it all the same.
With that said... it's your house, and if you like it, then enjoy it. There's no accounting for taste - and I mean that in a non-pejorative way; e.g. there is no hard math or rules for good vs. bad for a lot of this stuff.
As for the tile, I think it's very difficult to understand what you mean when you suggest "dark grey tile" with no other descriptors and we have very little to go on with respect to the current look or style of the house and where you want to take it.
I have a dark grey tile, charcoal nearing to black, in my kitchen, and I love it. But it's meant to look like stone, and has a very rough finish.
This is it up close:
And this is it in a very contemporary space:
....and I love it.
But there are, like, 50 shades of grey, cold greys and warm greys, polished finishes and rough finishes, smooth and rustic textures, etc etc etc. Grout color can make two identical tiles have very different feelings as well.
Overall, I endorse tile in basements, and I actually like them even in general living spaces where they are not often a first choice, but remember they can be a little cold underfoot... especially in a basement.