Why dont you try them sealed and supplement the low-end with some equalization?
You have 2 high excursion 15s, they have more than enough displacement to have crazy authority around 20hz.
All a subwoofer is doing is transferring electrical energy into acoustical energy...its just displacing air. The characterisitics of the subwoofer shape the response and when it should start rolling-off (high Qts driver dig deeper usually b/c they reach maximum excursion later and low Qts driver "spl drivers" dont dig as deep as HT drivers because they reach maximum excursion earlier)
soo...your Q 15 should be able to dig as deep or high as you like (within reason) its just a matter of what electrical equalization or motion feedback you apply to the response of the woofer which is shaped by its motor strength, Mms, suspesion stiffness, and Fs