Thanks, guys. I knew I could count on you. Especially Ken.
The opening is 9.5"h x 21"w. No room for a VP 150 between the doors.
I knew that it was too short for a standard M2 and never considered putting a bookshelf speaker on its side.
As johnk mentioned, it looks like I can fit the inwall portion of a M2 ow/iw in there with enough room for angling. I'm tempted to buy a pair on the closeout, although I can't really imagine what I'd ever do with the second one.
I have experimented with various angles of the VP100. Like I said, it's fine; I just wonder if it could be better on my terms.
