Recommendations for a new subwoofer?

And speaking of Rythmik (correct spelling ) Audio and electrostats: for those who want the ultimate sub/dipole combo, check out the dipole/open baffle woofer/sub that Brian Ding created in collaboration with Danny Richie of GR Research. Add that sub to your dipole loudspeakers (including any and all planars) for full range reproduction down to 20Hz. The only sub of it’s type (open baffle, dipole, servo-feedback controlled woofers) available.

Many thanks to all for your suggestions and advice. Unfortunately, in my setup, I only have room for a single sub, and my late, lamented REL S/2 filled the bill admirably. While I’m intrigued by the Rythmik subs, there seems to be an availability problem at the moment and, reading through the info on their website, they don’t recommend speaker-level connections for their subs, but right now, it’s my only option. Nevertheless, the F12 looks like it would be a logical choice for my system. Otherwise, I can see the upsides of the Elac, and I’m very tempted to go that direction despite the fact that it relies on an app. And ATC, as good as it may be, is outside my budget. 

Also, I did some online poking around about bass management but couldn’t find any useful information about how I might incorporate that into the system I currently have, and I’m in no mood to jettison my Modwright amp on that account. This is probably a totally different thread anyway, and I might post that question separately. 

Still haven’t ruled out a new REL, just have to settle on which one if I decide to go that direction. 

And a little postscript here, since it's too late to edit my original post: I mean speaker level INPUTS, not outputs. Oops. 

Speaker level inputs, not outputs. Oops

I think we all got that as a typo. 
Your observational reference about the general absence of line inputs…..

It constantly amazes me that hi level inputs are not offered as an industry standard on all decently priced (up market) subs. Why cut corners?  Maybe a deliberate marketing ploy by those manufacturers for a more customer perceived competitive retail price point versus the ones that do offer line inputs.  Hmmm

Well done Rel for taking the guess work out of it and offering line inputs AS STANDARD.

Footnote

Just to avoid any confusion, Myself and the OP are talking about high level SpeakOn (Nuetrik) speaker level line inputs.

Nuetrik connectors are regarded by professionals as the recording industries highest standard, for their robust construction and connective integrity.

@cooper52: Though Rythmik designer/owner Brian Ding prefers and recommends line-level connections, he offers his subs with a choice of plate amps that provide both high-level (power amp) and line-level (pre-amp) connections. My attitude on that subject is: Why add the distortion inherent in all power amps to the signal you send to your subwoofer amp? So that your sub suffers from the same imperfections as do your loudspeakers (the distortion of the loudspeaker’s power amp introduced into the reproductive chain)? Imo that’s a questionable approach, especially with a tube power amp.