Weak link

Your room is the most important component of your system. If it’s not treated, you will be fighting to get th best out of your other components.

You didn’t say what you didn’t like about the sound you are getting. If you don’t like the bass, room treatments will help. If your don’t like the soundstage/imaging, room treatments will help.

I would hold off on buying any subs until you treat your room. Your speakers are sized appropriately to your room size so you might not need subs. My new house has abut the same size room as yours, and when I bought new speakers sized for my room, I pulled 1/2 of my room treatments out of my room that were used because the other speakers were too big for the room. I have subs but I don’t need them with the new speakers. I still use corner tube traps and 1st reflection panels.

1st, document what you are actually receiving at your ears, seated position

cd with test tones

Here’s one on eBay

Amazing Bytes CD with 29 1/3 octave Test Tones

Sound Pressure Meter, with bottom threaded hole for tripod mount

Meter on tripod, at seated ear height, at listening position.

Meter doesn’t need to be that precisely calibrated, you will get more or less answers relative to adjacent frequencies.

Make enlarged copies of the insert with test tone list so you can write down the results

Go forth informed

IF you have any tone controls, you can measure the results, and you can make adjustments for your personal taste, perhaps you have age related diminished hearing of high frequencies.

also, great tools to accurately measure the effect of alternate placement/toe-in.

room treatments are always good, but i agree with the get-a-sub--your kefs are great but i believe only go down to 35 or so.

Sorry I  didn't list it, but I have a single REL S510 subwoofer.

Room treatments.  Hands down this will be the best bang for the buck.  Why spend money on better/more equipment when it will be conflicting with sub-optimal room acoustics????  Good luck and cheers