Small footprint subwoofer

+1 on the Kef.  A friend has one and I was quite surprised by how capable it is for its size.

The REL R218 is for home theater. For your stated needs, you need 2 x REL T5x subs. Stereo pair is so much better.

KEF KC92

I’d add the SVS SB1000 Pro to your list as it’s small but still has a 12” driver and goes down to an honest 20Hz (-3dB), and you can get two for less than the price of one of the REL or KEF subs mentioned — I’d definitely get two as it’s always significantly better than one, and you get a $50 discount for buying two. In addition it comes with a helpful app that makes integration much easier, and they offer a risk free, 45 day in-home trial including shipping both ways, so why not??? Read the reviews that are consistently very positive. And BTW, the only way these would overwhelm your room would be if they’re not properly set up (i.e. volume/crossover set too high), which again is made much easier with their excellent app. Just another option to consider, and best of luck.

I have dual REL T5/x subs that I really like but, on paper at least, they likely don’t compare to the Kef.  I went with the RELs due to cost as I wanted dual subs and the Kef is about twice the cost of the REL.