Well, you've done quite enough for me in these few short years of knowing you, so I am happy to build whatever!
Like I said, I'll check out the stone works place and see what I can find - figure its a good way to start a relationship for when I'm ready for new counter tops, anyway!
I do also have at least 1" thick walnut that could be used as the top as well. Been saving it cause I haven't needed anything that thick. It's not stainable but would make a lovely dark brown top, the grain is a medium brown in places.
I think Shady is right and the potential for discs rattling means the cabinet should be square, not slanted. I've got plenty of hinges, let me see if I've got enough fabric and can give them wood framed doors with a cloth center so the inside of the cabinet does not resonate. (he also uses rubber feet to secure the speakers)
My other idea is to use my Festool Domino on some mdf and create a heavy base with removable 3/4" layers to make the structure height adjustable. The dominos mean no tools/adhesives to connect each layer and the cuts are tight enough that nothing should shift around. So if I stack several pieces of 3/4" mdf, you've got a couple inches of adjustability - and no sand to clean up to provide a sturdy base!
What color are the speakers? Have you given thought to stand color(s)?