Which subwoofer should I get?

I would suggest any of the Rythmik Audio models, where it not for the fact that the company does direct-to-consumer sales only. I'm not sure what their audition policy is, so head over to their website to at least check them out. Then go to the RA AVS Forum, to see what RA owners have to say.

REL lovers often cite that company's excellent subs as being unique in offering high-level connections on speaker binding posts. That is not true; Rythmik offers a number of different plate amps, a few featuring high-level connections. The Rythmik plate amps offer one feature the REL's don't: a continuously-variable phase control, imo indispensible. Others include variable damping, and a choice of crossover filter frequencies and slopes. And all Rythmik subs feature stop-on-a-dime transient performance, thanks to the patented Rythmik Direct Servo Feedback control of the woofers.

I've used Rel subs for 20-25 yrs and have never seen a reason to change. I'm also a drummer who has sat near the bass amp for a long time. I know what a bass guitar (or stand up) sounds like. Rel produces that sound. But more importantly, its the ease of integration and the effect the Rels have on the main speakers. The sound from the mains becomes as if you had upgraded them. It give more of all the good things you now hear from them

I haven't listened to all the subs available. But I don't need to do so because of my extensive time spent listening to and playing live music. I can heartily recommend the Rel. One sub is adequate but two subs are better. You will need to size them properly for your room  & system.

Oops, "were" it not.

I use two REL S/510 for two channel.  My mains are rated fairly flat down to 28hz, but the subs still produce a notably fuller sound. They're punchy, not boomy. A few days ago, I moved my gear to a new rack. When I turned it on to test everything, I could tell that I had forgotten to turn the subs on.  The gain is set quite low as is the crossover frequency. They blend well with my mains.

I have a larger, ported SVS sub for home theatre.  It wouldn't work well for two channel. I'm sure SVS has better subs for music. Mine shakes the house when watching movies. 

Much better to get two subs than one, especially for music. I’d take two SVS SB2000 subs over one SB4000, and with their risk free trial period that might be a good option to try. If you liked one PB2000 then two SB2000s would be a significant improvement over that. The Rhythmic F12G offers big performance/$ but doesn’t have integration software like SVS but does offer speaker-level inputs like REL but much higher value as they sell direct. REL is expensive for what you get and better to go with Rhythmik IMHO (compare the specs to REL). Best of luck.

https://www.rythmikaudio.com/F12G.html