Which subwoofer should I get?

Very happy Rythmic 15 in sealed sub owner. Integrates really well with my Maggie’s and ProAc set up.

I use a SVS PB2000 in my office system with JBL L100 Classic speakers. I have a seamless sound and a single sub gives me excellent bass in a very large room. Their tuning app is excellent and works via Bluetooth to your phone. Once I set it up in Jan 2021, I haven’t touched the settings since. It like the Ronco oven, set and forget. I do wish I bought the sealed box SB200, but I use the port plugs and get an extremely tight and tuneful bass.

REL; set it up properly and forget about it.  Doesn't draw attention to itself, enhances the overall system, and no "app" needed.

The biggest flaw of almost all subs is the lack of a high pass filter for the main speakers. It's hard to do when you don't know the main speakers. But properly done a high pass will make the subs/mains twice as good because the main woofers have to do orders of magnitude less movement resulting in instantly recognizable better sound in the new bottom octaves of the mains. So I'd go for a sub with a high pass or find a way too add one your self.

Also, this is sort of taste. Closed box woofers are more accurate than ported ones(or passive radiators). But they don't sound as rich. Closed box subs are rare. Rhythmic makes them. There must be a few others.

I'm definitely getting some mixed messages about the utility of getting a sub that is app controlled versus the RELs without app control. That's ok, just want to acknowledge that there are differences of opinion (what else is new with audio?) 

If I go with REL, simply based on specs, it looks like a pair of S/510's which are rated at -6db @ 20HZ are obtainable on the used market for $4k for a pair. This is significantly less than the Carbon Specials and would allow me to purchase a pair immediately rather than waiting. As another data point, the S/812 would be around $4.5 - 5k for a pair.

If I decide to go for the RELs, what would you advise is the best model for my room and music listening preferences, factoring in the cost differences?

Thanks again to everybody!!