You have eight 6.5" woofers in the 1723 Towers vs a single 13" woofer. The woofers have twice the surface area producing bass as the sub, and that isn't even factoring in port output. Yes, the sub driver will have a longer throw, but that only really comes into play in deep bass. The sub will have more output below port tuning, but probably not anywhere else, assuming you are giving the speakers sufficient power. What is more, you are generating all bass below 80Hz at a single point in the room which maximizes room modes and puts you at complete mercy of the room's acoustics.
I would experiment with running the speakers full range along with the sub. That will give you more points of low-frequency emission which should help smooth out the low-frequency response, especially using Dirac. Those speakers should be able to produce pretty good bass, so I would use the sub to address room mode nulls that the speaker's placement aren't able to shore up.