I was going to center the B15 top-to-bottom and left-to-right and then put the tweet 12" above that.
At 33 x 22 x 16 it's close to a 3 Pi. So I was imagining a 3 Pi with a 15. I was going to use the same port placement (or the port the same height but centered.)
Using an online standing wave calculator I get:
multiples of 213 Hz for H=31.75"
multiples of 327 Hz for W=20.75"
multiples of 452 Hz for D=15"