Which subwoofer should I get?

I have two RELs connected via high level and they are outstanding.  Super easy to integrate without the use of an app, just your own ears and maybe some measurements using something like REW.  The measurement step isn’t strictly necessary I just wanted to know.  REL has plenty of videos you can watch to help you sort it out.

To all: once I again, I really appreciate all the input. 

@ Rick_N: helpful to know your experience with the SVS SB subs. 

@ gkelly & jastralfu - good to know the REL's are easy to set up.

Another question for everyone: on the REL speaker matching page, they are recommending two T/5x's as optimal for my setup. BUT.....their speaker matching page does NOT have the A5's  as an option, (I chose the S3's as closest match) and they do not have a choice for audio only. 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? Is there a REL speaker that you think would be the sweet spot for my room and system?

FWIW, I listen to: acoustic and electric jazz, rock & pop of all eras, and all types of classical music. Oh yeah, occasionally, modern country and bluegrass. 

Hello @mwsl , I demoed both and bought the REL Carbon Specials.  I have a pair of Carbon Specials in one system and a pair of Carbon Limiteds in another.  One pair I run wirelessly with REL wireless system which allowed me to experiment with placement.  Despite some saying their controls are limiting, I am able to dial them in to where they literally disappear, yet add bottom end, air pressure, enhance soundstage and generally improve things.  My speakers are fairly full range (Vivid Giya G2.2s) yet my system clearly sounds better with the subs.  Once dialed in, you completely forget about adjusting them and just listen to the music.  Better than another remote control I think.  People often think my amps are my subs...and then ask where the subs are.

I have two REL S/812s in my system.  I like them very much.  My one complaint would be the lack of an app or more specifically, a remote control.  I find it impossible to tune the subs into the system as one size does NOT fit all.  Sure they are fine for the majority of the music I play, but I'm constantly finding way too much bass when a recording has piled it on.  A good example is Herbie Hancock's Dis Is Da Drum. The subs would dance across the floor if I didn't jump up and turn them down.  Would a remote have been that hard to incorporate, given the price of these units?  Cheers.

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