Which subwoofer should I get?

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!!

I realized I have a separate but related question. My integrated has two subwoofer out RCA connectors. The manual states only the following about them: "This output allow you to connect an active subwoofer or a second power amplifier." 

Does anybody know if these are "high" or "low" level outputs?

Given that the T/5x's are only rated -6dB at 32 Hz, I wonder if this is enough low end given my speakers and room? On the other hand, I'm a little concerned that a pair of Carbon Specials may be overkill for my situation (and a lot more expensive). Thoughts about this given my room size and need for music only?

Some thoughts — a sub that’s down 6dB at 19Hz at the price of the REL Carbon Special is nothing “special” at all when many other very good subs at less than half the price go down to an honest 20Hz and below.  A lot of spatial cues that get unearthed down there will be lost by many REL “subs.”   Many don’t realize that while not many instruments play down into the 20Hz region, there is still important recorded musical information that resides down there that plays a big part in improving imaging and an expanded 3D soundstage.  I wouldn’t worry about overkill for your room — just buy good subs that can get you to an honest 20Hz (-3dB) and if properly dialed in not only will they not overpower your room, they will work much less hard and with less distortion than smaller subs.  As for SVS subs being too sluggish for a 2-channel system — horse hockey!  I and many others here have gotten excellent results with SVS subs when properly dialed in, and all reviews back this up.  But you already got pleasing results even with a ported PB2000 so you probably already know this.  Just my experience and $0.02 FWIW. 

I had a similar dilemma. I was matching to Revel Studio Ultima 2’s. I went back and forth between buying REL or Rythmik. I read and read, listened some, but never sub for sub with my own Revel speakers, in my own listening room.. I ended up buying two Rythmik  F12SE’s. I did get a discount when you buy two at the same time.
I’m driving them with an Odyssey Kismet Amp and Hegel pre-amp. I suggest you talk to the folks (Enrico) at Rythmik directly. One note, the guys at Rythmik tried to steer me to instead buy two E15HP2 subs. I didn’t listen. I regret not taking their advice.

I could have bought REL, the choice wasn't about the money. It was there appeared no good reason to spend more money on the the RELs. BTW..today 1/2/2025, Echo Audio in Portland Oregon has two REL 810 subs for sale in very nice condition for @$3700 for the pair. If this would have been in play, I don’t know it I would have passed that deal up to buy the Rythmiks..