Research for subwoofer

Given the volume of your space and bookshelf speakers a stereo pair of subs would make a major improvement in the sound.  If you're only focused on 2-channel I would also recommend REL.  Once setup properly, these will provide seamless integration to the point where you will only know they are contributing if you switch them off.

Best of luck...

AudioKinesis Swarm 4 sub array: https://www.audiokinesis.com/the-swarm-subwoofer-system-1.html

Below is an REW spl graph (1:12 smoothing) showing useful output to 20Hz in a 28 x 18 x 8 ft room with the Swarm.

While many subs can get you to 20Hz, multiple subs of any make will get you there with a flatter frequency response and fewer room modes. More data available on my virtual system page.

++2 Rythmik - any of them...  I also have a SVS 3000 micro - but it is for a unique application.  I have owned REL - very good - but Rythmik is very impressive and musical/articulate.

@tcutter that scale is laughable. smoothening for bass should be in the 1/24 1/48 range

@kuttler, Do you have line (low) level (preamplifier outputs) on your receiver, integrated amplifier or preamplifier?

If not you"ll want to mention this during your search.