I think there was talk about that and a 3-way design in the Studio Monitor development thread, but ultimately ended up not being worth the effort. I think it was intended to use a pair of the same woofers found in the X-LS/X-Static?
But I imagine you could probably design build a set that incorporates a 12" side-firing, sealed, servo sub, so long as it has enough internal volume & bracing to support it, plus you'd get solid response down into the 20s.
If you wanted a more "traditional "box" look it might be possible to have one with the mid woofer on the right/left of the OB tweeter, and the sealed sub below it. Or the standard T-M configuration with the sealed sub on the right/left, for a more horizontal orientation. Tho I imagine both of those ideas will affect tone response for the mid woofer & Sub.
But in the end, the easiest/best solution ended up just being the Studios on top of a pair of H-frame Subs.