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.