I’ve had a few pretty good subs, including an early HSU (in a cylinder enclosure, similar to your SVS), a KEF B-139 driver (the one Dave Wilson used in his original WAMM) in a transmission-line, and the bass columns of the Infinity RS-1b loudspeaker (six 8" servo-feedback controlled woofers per side) I used to own. I now have a pair of Rythmik F15HP subs (each containing a 15" servo-feedback controlled woofer and 600w amp), as well as the very unique Rythmik Audio/GR Research OB/Dipole sub, which is not for everyone. Rythmik is becoming popular with Maggie owners, they’re that good. I also have a pair of Magneplanar Tympani T-IV’s, which have two 16" x 6’ panels per side just for bass! They produce a very different kind of bass than boxed subs do (leaner, very "percussive". They do drums better than anything else I have ever heard), and were a favorite of Harry Pearson.
If you want to stick with SVS, their SB13 Ultra (13" driver in a sealed enclosure) is a great sub, priced at $1600. SVS has just introduced the SB16 Ultra, so used SB13’s are popping up.