I've been saying for years that the servo OB bass is the best you can get. It's still true. To get the best sound, you must use it in stereo, you must place them fairly close to your main speakers, you must have them 3 feet from the rear wall and you must point them firing forward toward the listening spot.
What you will get is the cleanest, clearest bass you have ever heard. And at the same time, bass that can punch you in the chest.
As for why it's not more widely used, well most audiophiles are timid and tend to shy away from anything that's even a little different from their past experiences. Servo OB subs, even though MUCH better than box subs, just don't have the history or marketing that boxed subs do. Plus (because of placement requirements) have WAY lower spousal acceptance factors. A box sub, even a big one, at least can be tucked away into a corner. OB subs cannot.