had the question other day - 2x12 THX Ultra or 13 THX ultra. roughly same price.The Monolith 13 THX Ultra is 1180€ at the moment and it's in a whole different league frankly.
The 12" that I got is still comparably powerful (10dB higher SPL at 20Hz with half the distortion and 3dB lower SPL at 40Hz, but again less than half the distortion) and 690€.
From the reviews I've seen so far sealed really only means higher distortion in the lowest frequencies, I don't get the appeal of that.
I could afford 1 12" THX ultra now and add one later, not sure if I should. The neighbor downstair would probably be unhappy about it