Subwoofer Choice

I am in the process of building out my system. I have the end goal in mind, and ticking off the equipment as I get them. I do need some advice in the subwoofer category.

I am currently driving Monitor Audio Silver 500s with a Michi X5 integrated amp (600w @ 4Ohms RMS). This is way overkill for these speakers. But my purchase (coming in August) will be the Focal Sopra No. 3s.

That being said, I am looking also to add a subwoofer and I’ve settled between two (2) REL T/9x’s or one (1) REL S/510. T/9X is $1,249 vs S/510 at $2,749.

There is a REL youtube video from their chief designer comparing (briefly) the two and actually stating that "just because one S/510 costs twice as much as 2 T/9x don’t think you can equate them with 2 vs 1". I paraphrased a bit.

But the below is only 3min long please watch and let me know!

What are your thoughts. Obviously 2 subs are better balanced than 1, I get that. But given the specs here am I better at buying up for the better 10"?

Listening room is carpet, 16’ x 24’. 384sqft. 8ft ceilings. What info did I leave out?