REL T/9i or S5

What speakers do you currently own? Look at REL’s subwoofer matching, they are pretty specific. I can tell you the T9i is a hell of a sub for the $. If you can do two of those, depending on your mains you would be happy. FWIW I have an S510 and plan to add another soon. My room is a bit bigger than yours...

I purchased a T9i for $750 and later, an S3 SHO for $1000, both new, from magnolia.

I’d take 2 T9i’s over a single S3 SHO.

The S3 SHO does seem to have quite a bit beefier driver.  If you can do 2 S3 SHO’s that would be pretty sweet.

I’m currently using a T5i with great results in my small listening room.  I will get at least 1 more if not 3 more eventually.

Rels subwoofer guide is very useful.  You can easily get too much sub for small speakers.  You won’t be able to properly integrate the sub if its too large for your mains.  The S3 SHO with Totem Arros was a mess. Bass went from too quiet to overbearing with one click on the volume knob. Something to keep in mind!

Also, 4 T9i’s might be better than 2 S3 SHO’s

One of the nicer subs I found in that range the Rythmik F12G.  I owned a S/3 for 4 years and tried T-9 lent by a friend for 2 weeks.  The Rythmik is worth looking at.

I have a pair of T9i's that I use with my Tannoy Stirlings in a room a little bigger than yours.  I can't speak to the S line, as I never got the opportunity to hear them.  I'm sure they're wonderful.  But I find the T9i a *highly* musical sub.  Very fast and tuneful, which is what I want in my bass.  I want tones from a double-bass, not boom or rumble.  They certainly could overpower a room and muddy up, but they're so adjustable it's easy to avoid that.

There’s no substitute for quality the S line is considerably  better. All the best