My first recommendation would be 2 subs at a minimum. I've had the Swarm and it would likely work well with your room. Acoustic studies (see @rauliruegas comment on my post-Subwoofers are a superlative room treatment) demonstrates that 4 subs are the optimum number. The Swarm enclosures are small enough that they will likely fit well in your space. These little guys can put out bass, but by virtue of the noise cancelling properties of 4 staggered subs, will make your overall sound much better. Much much better. Duke Lejuene, who designed and manufactures the Swarm, is also extremely knowledgable and will help you with the set up and adjustments as necessary, as he did for me. He will also help you decide if his system will work with yours. I did switch them out in favor of 4 B&W DB1B's, though. My Magico S7 speakers have 3 10" woofers apiece and the the Swarm has 4 10" drivers-not enough for my system, but likely more than enough for yours. There is contact info for Duke on the Audiokinesis web page. I'm excited for you-a pair or 4 subs will do a lot for your system once set up properly!
Subwoofers - Final Thoughts with Martin Logan ESL 13A
I’ve narrowed this down to 3 choices (I think) and was looking to see if anyone has had some experience with pairing these subs with ML ESLs? I can’t try and return because they are mostly secondhand purchases, so hoping to get it right the first time!
Force Cancelling subs have been highly recommended, so:
KEF KC92 (or KF92 older model) 2, 3 or possible 4 of these:
https://assets.kef.com/product-support/kc92-subwoofer/KC92_info_sheet_EN_V4_20240124.pdf
Martin Logan BalancedForced 210 - Maybe the best as they match up nicely, but I could probably only do one of these as they are HUGE. I have only one spot I could put one as far as the manual's recommendation goes. Martin Logan says these will cover 3000 sq. ft. My room is only 500 sq. ft. Two maybe overkill, but some say, "2 subs or no subs" Maybe I could put another addition on the house :-(
https://www.martinlogan.com/en/product/balancedforce-210 - click on Specs/Lit
Lastly, The Swarm Subwoofer System - not sure how well this would work with my ESLs.
https://www.audiokinesis.com/the-swarm-subwoofer-system-1.html
Any advice would greatly be appreciated (as usual).
Thanks, and all the best of the season to you all!