I'm thinking about getting an OB subwoofer to fill in below 60hz. Due to budget and space, only a single mono sub is being considered. My question is this - how loud will a dual or triple OB sub play? My main speakers have a 5.5" woofer, so very high levels, say above average 90db is not my concern. I don't listen at high levels often, anyway, due to very young kid, soon kids, plus hearing damage that makes threshold for pain quite low. Subjective opinions on loudness can vary greatly from person to person, so I was hoping to get some SPL numbers as a rough gauge.
My listening is 65% music, 25% online media, 10% movies, and rough room sketch is below. Vaulted areas have a 30 degree slope starting at 8 feet at the lowest. No vault areas have a drop ceiling, 7' high. Walls are denoted by solid black lines, light lines are knee walls.

I currently have an old GR Research 12" with passive radiator sub in the front left corner. If need be, I suppose I could keep it to fill in the bottom octave, but it seems a bit silly to spend all this money on an OB sub to only have it play from 40-60hz. I've also never really heard any output from the old sub at 20hz. Measurements at main listening position shows it is -20ish db at 20hz. 25hz is solid, though.

I cannot place the sub in the middle between the speakers as I have seen recommended elsewhere for single sub mono use, as that is where the TV is. Anyone have any measurements to share?