Subs?

For sure not one sub.  It may make your system sound worse-it did for me. Two subs at a minimum, 4 being best.  As has been previously mentioned-the biggest effect of adding subs is how it cleans up the sonic clutter in your room. The second effect is a tighter more natural bass. The audiokinesis swarm is a great value and would likely work well with your speakers.  I had one in place for several years, until I realized my Magico S7's with 3 10" woofers each were running over the 4 10" subwoofers in the swarm. I switched to  4 B&W DB1D's and the improvement was astonishing.  If you don't go with the swarm, I would recommend that you chose a sub with DSP capabilities.  Again, at least 2 subs!

I'd have a good look at the Martin Logan BalancedForce 210, a pair of them.  They perform great and look just as good.  I really liked their older subs and I have owned them all.  I still have an old Grotto that I use daily in our family room system for TV/ movies and it's great.

@goducks44

Don’t waste your time with all these hoity-toity over priced underwhelming boxes... (2) 18" Eminence Tour Grade Subs powered by a Crown macrotech 2400 with a X-over, the rabbit hole is closed...game over.

Go Ducks! 🦆🦆

I'll throw in on this one...SVS makes a whole line of subs that rattle the rafters. They come with very thorough software application to help integrate into your system and if you choose correctly you'll find they offer very low distortion and enough headroom to satisfy anyone, especially if you intend to do multiple subs. They have a just recently released model in the 17" size that looks very impressive and according to what I have read about it sports the lowest distortion of any sub currently available that I am aware of. They are not cheap but I can say with confidence they are sure worth taking a listen to. They are readily available thru Crutchfield and other places too as well as direct from SVS. I have a couple of their smaller subs  in my music room and they do a fine job of filling in that bottom octave.

@goducks44   I have a similar sized room and my mains are the Magico S3 II's.  I also went with the Audiokinesis Swarm Sub system as has been mentioned many times in this thread with amazing results.  They truly transformed my system.  I worked with Duke to get custom stained veneers to match the furniture in my dedicated listening room (Stickley Mission Oak) on the four subs.  They integrate beautifully and the two that are in my line of sight double as plant stands!  Can't recommend them enough!