Recommendations for a new subwoofer?

Oh, and an afterthought: if I end up with a sub that has speaker level outputs but not a speakon connection, what would you recommend for cables? Are there speaker cables that are more amenable low frequencies, or would any good cable do the job? Thanks.

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. 

OP here. Wow, this has turned into quite an education. Much to consider here. I’m especially grateful to deep_333 for the detailed explanation of low frequencies. Despite being a (mostly retired now) professional musician, I hadn’t really paid attention to the octaves issue (i.e. it’s a full octave from 10 to 20 Hz, and another full octave between 20 and 40 Hz, etc.) even though it’s something I’ve known since back in elementary school. 

And also, acknowledgement to m-db for pointing out that my Modwright integrated has pre-outs, which simply escaped my notice up to now. (Doh!). I actually tested it out this morning with my headphone amp just to see if it was functional, and yes indeed, the pre-outs work just fine. So that opens up the field quite a bit.

Rattling the windows and shaking the floor is definitely NOT what I’m looking for in a sub. That’s why a 10" driver is pretty much all I think I’ll need. Just needs to be musical and well integrated, with good "composure," meaning clean and well-defined. I suppose the words "fast" and "accurate" are probably more commonly used to describe these qualities. 

Right now I’m leaning towards the Elac, partly because of availability (Rythmik and MJ Acoustics are equally tempting but seem to be harder to come by at the moment) and partly because of the rated frequency response going down to 18 HZ. (The REL bottomed out at 24 Hz). Haven’t entirely ruled out REL yet, though. 

Still thinking...

Doing a bit of calculating here, just for the fun of it. Modern concert "A" has been standardized to 440 Hz (though this varies with different orchestras, some tuning a bit higher, like 441 or 442). An octave below that (220 Hz) is the top line of the bass clef, and an octave below that (110 HZ) is the bottom space of the bass clef. Halving that again to 55 Hz, we find ourselves 3 ledger lines under the bass clef, which, on the double bass, is the open 3rd string (though, for those of you who might not be familiar, the double bass is always notated an octave higher than it sounds so it can be read comfortably on a bass clef--that open A string would be the bottom space) and if we halve that to 27.5 Hz, that’s quite a bit lower than any orchestral instrument can go. The double bass bottoms out at about 41 HZ, so a subwoofer that goes down to 15 or 18 Hz is REALLY gettin’ down there. 

Here's an update: based on reviews, suggestions here, availability, price, and my ability to make a deal I'm comfortable with, I've ordered an Elac Varro RS-500 which should arrive in a week or two. My plan is to connect this via my Modwright KWI 200's pre-outs, and we'll see what we get. I'll post my impressions when I've had a chance to put it through its paces. 

BTW, for those who may have missed it, I posted my question about high/low pass filtering (q.v.) separately, and that turned into an eye-opening discussion which you may find enlightening. I certainly did. And no, it's not gonna happen in my system.

Here's an update: the ELAC Varro RS500 sub arrived yesterday and is now going through its paces in my system. Right out of the box, I'm kind of surprised how different this sub sounds to my previous (now deceased) REL S/2, but I'm certainly not disappointed. ELAC's app has proven extremely useful in getting the sub to integrate with my main speakers (to remind you: Ryan R-610's), though I've done just a bit of my own tweaking in terms of volume and placement. Right now I have the low-pass filter set to 55 Hz, which seems to be a pretty happy meeting place. I haven't yet delved into the app's parametric EQ utility, but hope to do so soon. The utility I've only played with briefly is the phase-angle adjustment. It seems perfectly happy set at 0 degrees for now.

The ELAC definitely delves deeper into the low frequencies than my REL did, but what I'm noticing immediately is that its presentation is not quite as tightly defined as REL's. However, what I am hearing seems to be a more melodic rendering of these low frequencies, so it's not necessarily a bad thing. 

Like all new speakers, this one will probably require some significant playing time before it finds its true voice, so I'll give it a few weeks and see how/if it changes and post an update then. I have it connected via line-level RCA cables. It would be very interesting indeed to see how it behaves through speaker-level input, but that will have to wait until I really feel motivated to do it, as I'd want to invest in some decent speaker cables to try it.

Once again, thanks to all of you for your opinions and information in this thread. 

Here’s the update I promised in my previous post: after living for a few weeks with my new ELAC Varro ES500 which I think has probably had enough time to find its voice, I’m liking it quite a lot. My first impressions were, as I said, that it was startlingly different from my old REL S/2, and this is still true, but what I’m hearing is added warmth, though not muddy at all. It seems to blend a bit more easily with the rest of the sound spectrum. The REL held bass frequencies in an iron grip, which certainly has its appeal (I lived with it for 10 years, after all, so...), but so far, I’m liking the ELAC better. 

My musical tastes are a bit all-over-the-place, though predominantly classical, as I am a (mostly retired now) professional musician. But I still revisit my misspent youth a lot, which includes the popular music I grew up with (think San Francisco in the late 60s) across the genres of rock, prog-rock, and English folk. I’m also a great admirer of Broadway shows. The latter, when well recorded, are particularly good for showing off the attributes of a system, and here I’m finding the ELAC sub to be a very entertaining and appealing addition. 

At the beginning, the sub was connected via line-level cabling, but I couldn’t resist the temptation to try out the speaker-level option, so ordered some cables from Blue Jeans Cable just to experiment with, and lived with that configuration for a week or so. A few days ago I changed back to the line-level connection, and by a very thin margin (which might simply be my imagination at work) prefer things this way--just a tiny bit fuller and more musical. 

Hope some of you find my experience of interest. And again, your input here was greatly appreciated.