Bookshelf/subwoofer combo

maybe here >>

I think a crossover is the most important is a system with subs. Lots of digital preamps have subwoofer control. If you want to use analogue system sublime acoustics makes some very good ones. You really need to highpass the subs for best integration. 
 

https://sublimeacoustic.com

Hopefully I’m wrong, but does Frank 009…

@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.