Suspended wood floors can be a problem with some tables and especially speakers for that matter. Just a guess as I'm not a structural engineer, but if you're still in the framing stage, I might double up the 2 x 4 ( or 2 x 6) framing in the area below the wall where the Amadeus is going to be mounted. Probably double up framing upstairs in the area where the you're going to mount the shelf as well. Now if everything has been drywalled by now, never mind. At this point I would study the various wall mount systems (or even a DIY approach) and determine which would work best in your situation. In addition, even though a corner location is structurally favorable, I've heard many times it's far from an ideal location for a TT due to standing waves. I'll let the experts chime in here.
Hey Mick, how have you been?
We've just broke ground, so nothing's set in stone just yet, though I have been talking to our builder about my needs for the room, and you've pretty-much hit upon many things we've considered. He's a long-time family friend, so is totally in-tune with what I'm looking at so the room's as close to ideal without getting crazy with the budget.
Our current house which we're about to close on was never ideal for audio, yet it sounded quite good and measured well. The floors were less than ideal, yet you could jump up and down forcefully next to the turntable and never affect it in the least (i.e., footfalls were never much of a problem, though theoretically it should've been).
Anyway, I really am just looking for ideas on certain brands that might work; and if the shelving unit, itself, is too small (not the mounting hardware, per se), I could always make one from maple or bamboo.