Awesome!
I had a flash a couple of minutes ago... If this is what's happening, I officially have super powers! An add on top cabinet for a mid woofer for the Sierra-1 (with crossover, of course). Let the rumor mill begin to turn! Logic behind the guess: 1) Doubt it's a tweeter upgrade, 2) doubt it's a woofer upgrade, 3) the cabinet must remain the same, so I don't know, for some reason the design of the tower with the mid on top and likely inner volume for the sealed mid, seems doable for a Sierra-1, so while I wish I'm right, I hope I haven't spoiled anything!!!
(if it's right and surprise must be kept feel free to delete this post!)