I've got my sub sitting atop my FC-MOD right now.
I had it set up with the AC directly opposite the mouthpiece, with a cloud buddy pointing in the same direction, and the sub placed on the outermost joint.
I figure this is as tippy as it could possibly get.
With the sub facing back toward the mouthpiece it's fine - stable on its own, not wobbly, but will probably topple if bumped with moderate force.
With the sub facing out it'll tip.
Imo it's a fine setup balance wise.
I haven't used it yet, so no function report just yet.
Edit: