Yet another Sub Question

what does the REL website speaker pairing suggest?

9x

Well the REL site does not have my QUAD Z-4.  But the Z-4 are in the same class as say Focal 936 or 948.  I had the 926 prior to the Z-4.    All the Focal Aria series result in a 9x suggestion for my room size.   Thats how I ended up with one. 

I was afraid that REL would not have your speakers. Wish they would up their game and add more. Now I guess the question is, do you plan on upgrading your speakers in the near future. If the answer is yes, then Maybe looking at the 510s would be an option. But still at the end of the day you gonna wanna have two. At least with the 510s you’re on your way to a stack array. Now we’re cooking with gasoline!⛽️ 

Already half way there, get the second T9x. I have two, easily beats any of the single sub setups I tried over the decades. Far better integration with main speakers, simply an extension rather than an add on. Far more forgiving of sub placement and listening position. Sound stage improved noticeably. I cross mine over at 32hz, 3 notches up from minimum volume level, didn't really need much with my extensively modified Klipschorns. I've also played with various diy cabling and configuration of that wire with my 300B monoblocks, you can do much better than the stock REL cable.

You can make a single sub work if you have freedom of placement. Most probably, you will need long cables running to the subwoofer location that works.

Get two subs!!!