Great documentation, design, and build :TI don't normally listen to stuff over reference anyway. I couldn't until this second horn... the room is open to the rest of the basement, which in turn is open to the whole house. Takes a lot of SPL to fill all that up. That's why I was so unhappy with one 18" in a vented box.
My house seems quite similar, and all the subs I listed below (18" & two 12"s) do a good job, but nothing like others have with a lone 15" in a small room. I've vowed to use an infinite baffle for any future house to achieve desired spl without taking up space, but thanks to you, a tapped horn built as a second row riser will now be a definite possibility.