Well, since you’re in research mode maybe take a look at a pair of SVS SB1000 Pro subs. Before you panic, these are only around 13” square so visually unimposing yet will still get you down to 20Hz (-3dB) and include integration software. Most people here much prefer two subs to one and for very good reason, so if you can possibly sell two small subs over one bigger one to the wifeboss I’d at least give it a go. Another plus is that a pair can be had for as little as $1150 (depending on finish) and come with a 45-day and totally risk-free in home trial that includes shipping both ways.
https://www.svsound.com/products/sb-1000-pro-subwoofer
If ya gotta go one sub I’ll reiterate looking at the Rhythmic F12G (the one with the paper cone) and buying a used DSpeaker integration device like this one (retails for $1199 so at $600 or less here it’s a good value IMHO), but at upwards of 16” square the Rhythmik could actually be more visually imposing than the SVS subs — always trade offs.
https://www.ebay.com/itm/145216049992
Rhythmic subs offer very high performance/$ but lack integration software hence the DSpeaker unit, which is excellent, and you’ll still be under $1800 for both.
That said, I think you’d be very happy with a JL Audio sub as well although it’s relatively a bit pricier, and the Arendal sub looks very interesting as well but I’m just not familiar with it. I don’t consider the REL T9/x to be a true subwoofer as it’s down 6dB at 27Hz so it’s more like a regular woofer, and there’s very important musical content below 30Hz that can provide significant improvements in areas like imaging and an expanded soundstage (subs are not just about added bass — not by a long shot). Anyhoo, as you’re in research mode I thought I’d just give you more options/info to chew on and hope it helps at least somewhat. And welcome to Audiogon! Great to have you here.