Older vs. newer REL subs

I'm following this thread with a lot of interest. I have a T9i sub and have been contemplating whether to get another T9i (used) or save more to move up the line. The problem is that I bought the T9 in white to match the KEF LS50s which were also white. The KEFs are now gone, and the white sub doesn't go with the aesthetics of my room anymore. So the question now is whether I sell the white T9i and buy a pair of black T9is, or buy a single S/510 for now, and buy a second one next year.

I have read many posts about the S line being better, but don't quite understand how specifically. I called REL customer service and they feel that a pair of T9is are more than sufficient for my needs (not a bass head, no EDM or hard rock/metal for me). But I do wonder what will I be giving up by not getting into the S line? Hopefully, someone who has actually moved from T to S will chime in soon.

BTW, one thing I can tell you is that the T series is absolutely better than the older REL subs. I had the REL Storm III about 5-6 years ago, albeit not in the same room or even the same equipment/speakers. While it had adequate bass and pressurized the room nicely, I yearned for the bass to have more punch and texture. The T9i definitely delivers on this aspect. The bass is more focused and 'well put together' if you will. You definitely feel the improvement when listening to classic rock for example.

OP - sorry, didn't mean to hijack your thread. If this sounds like too far removed from the topic at hand, I can start a new thread :)

@missioncoonery -- I hope you can shed some light on what aspects of music improved when you moved up from REL's T to S line? My room is 20 x 15, when I called REL customer service they said T9i will be good enough (I already own one) for my room and listening preferences. But a lot of people have told me that there is a distinct advantage when moving up the line regardless of room size. Is it about more bass or an increase in punch and texture or both? TIA.