The sub you have is excellent. I would use it in the neutral mode and not extended, and certainly not sealed. It is an excellent sub, and if you get something else you have a very good chance of getting something worse and not lateral.
A sub is only part of the equation, the speakers it is mated with are the much larger part of the equation. I am pretty sure subs are blamed for ills they are not responsible for. The sub is the easiest of all the speakers to design, the rest much more difficult. The tightness of bass actually comes from the accurate reproduction of frequencies way beyond the sub range, and actually out as far as 4KHz! So the FRs, and especially tuning and phase response of the other speakers, are vitally important in the illusion of truly accurate bass. To be honest that is why subs and speakers are optimally designed as a totally integrated system. That is the way I design my systems.