Bookshelf/subwoofer combo

@james633  The subs come with a low pass fliter.. If the main speakers start to roll off at around 50hz and then set the low pass filter on the sub to 40hz you should be in the ballpark.

REL also recommends connecting the subs through the high level connection and DO NOT  use the pre-amp sub out. Go direct from the amp to the sub plate amp. This way you pick up the dynamic character of the amp.

Also, you don't have to high pass to the main speakers either. They can be run full range and cross the sub low. There really is no reason to introduce another piece of equiptment.

I will suggest something else, having lived with subwoofers for several decades.

I will suggest that ATC SCM40’s don’t need subwoofers in that size of a room and will work better alone. Soundstage and coherence will be much better. I use SCM40’s in a 20 by 22 foot room and never felt the need for subwoofers.

If you do go the subwoofer route, I would suggest getting two, as stereo imaging and coherence will be better, it’s easier to integrate them in. But then that is a lot of complexity and expense.

Joseph Audio is highly thought of, this is at the top of your budget new. They uniquely use Richard Modafferi’s patented Infinite Slope crossover.

https://www.josephaudio.net/news/2019/4/6/introducing-the-pulsar2-graphene-the-biggest-little-speaker-on-earth

https://nowlistenhere.net/speakers/joseph-audio-pulsar2-graphene

Have you considered used? These, prior model, are in Greece. No need to buy/ship those stands, get a discount and buy stands where you live.

https://www.south-sounds.com/en/speakers/626-joseph-audio-pulsar.html?refsource=hifishark

I would avoid buying from Greece, tariffs and customs could be a nightmare

I recommend two or three subs.  Rhythmik F12 or the bigger dual sealed or F15 would be great in that space.

I agree with those that say maybe tower would be better.  Some used tektons could be amazing for that space.  Like used moabs or double impacts.  Could get new but some people don't want to support them after the battle with Erin at Erin's audio corner.  Otherwise keep an eye on Perlisten or Arendal monitors/smaller towers!!

Best of luck to you!!