What I'm struggling with is that 800 Hz corresponds to a pretty short wavelength, eg 1/2 wave is maybe 8-9". Seems too short for the spike to be a room or speaker placement issue unless maybe you have speakers ~1.4 feet from a wall ?
Or your DSP has become self-aware, starting in the 800 Hz band.
Mojo - LOL !!
I don't *think* the skylight dimensions are right for it to contribute to the 800 Hz spike but I would be tempted to temporarily stuff it full of fiberglass or rockwool insulation just to be sure. If your 800 Hz spike becomes a suckout then you just take half the insulation out.
It does sound like the kind of room we would ask for more details about just because it would be fun watching you try to draw it