That depends on the "subs".
Each SV6500 has 3 x 6.5" woofers, so pair of towers = 6 x 6.5" woofers = 200 sq. inch surface area of woofers. That's equivalent in surface area to a single 16" woofer. Of course, other things like total cabinet volume, amp power, driver materials, etc., factor in.
It won't outperform bigger subs like the RBH SV1212 and other bigger subs. But 200 sq. inch of woofers can produce a lot of bass.
Bottom line, you can turn the SV6500 tower into a passive monitor + active subwoofer system.