Old and New-- adding Rel s510 or 812 Sub to heritage (older) McIntosh system-- Worth it?

Appreciate advise on adding a subwoofer to my older McIntosh system.  That system  consists of a 2125 Amp, C28 pre amp, Mc101 Equalizer, XR5 Mac speakers, and a Bluesound Node X (new) for streaming from Tidal (jazz, classical, blues, some folk).  Room is 21 x 15 x 8.5.

I'd appreciate advice on two fronts:  will the Rel subs (S510 or S812) significantly add to the depth and spacial experience of the sound?  Not interested in bass boom, but increasing the spacial sense and lower bass definition of sound (which seems to be Rel's wheelhouse).  And secondly if so, would a single S510 be sufficient in my room size or should I look for a single S812.  Don't want dual subs at this point.

Just one thing to add-- in my 40 plus years of moving this system to various homes and rooms, this is the first time (since my old direct disk phonograph days) that the system has come alive, primarily due to the moving of the speakers away from the walls, and having a room which really sounds good.  The magic of two channel audio is so nice, with disappearing speakers and the experience of live sound (especially with acoustic perfomances)  I've auditioned much higher end systems in dealer showrooms which I did not feel carried the warmth and realism that I now experience.  The final piece would be to get a bit deeper and richer soundstage which many reviewers report from a properly set up subwoofer.  Thanks in advance.