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...