Research for subwoofer

For your room size I would suggest the REL S Series if you go with REL.  They are expensive though as in >$3k.  You’d likely be better off with some Rhythmik subs.  Easier on the pocket and excellent performance.  I have a couple of RELs (T5/x) for my small space and they are outstanding but would get swallowed by your room.  The T9/x would might be sufficient with two or more but they only go down to 27Hz.  Something like the S550 goes down to 19Hz.  The Rhythmiks play low like the S550 but at a fraction of the cost.  

@kuttler https://hsuresearch.com/collections/collections-true-subwoofers-subset-collection/products/vtf-tn1

Unless my Google is mistaken this is the second thread this week where a person has A Luxman 509, a $10-$15,000 integrated amplifier paired with budget speakers. Though I do agree that getting a subwoofer is in order finding speakers that will dance better with that amplifier would do you a whole much more good at this point. Just my opinion maybe I’m wrong about the price of the integrated.

Which is which? Sub-Bass Systems was the term Richard Edmund Lord the R.E.L. of REL used for his systems.  

REL subwoofers (I still like our original term Sub Bass System for its more complete description of what we do) 

This admonition was from a website brochure by the current owners some time ago. Another consideration, the company matches their systems to a list of speakers.

-3dB subwoofers are positioned and adjusted to integrate with most any speaker and room. Update your speakers and simply readjust your subwoofer.

Despite these differences adding a sub-bass woofer system offers setup simplicity and a reasonably enjoyable presentation at the expense of the surprisingly stunning extra low frequency found on many recordings and programing. All the best with your search.  

Given the volume of your space and bookshelf speakers a stereo pair of subs would provide a major improvement.  If you're focused on 2-channel only REL would certainly be my first choice.  They really integrate well, when setup properly you can't tell the subs are there until you switch them off.