Yet another Sub Question

If you have one and like it get another, or sure upgrade a level or two. The second extends the soundstage, can make a big difference.

However, don’t overdo the volume. Typically they are set to very low volumes or they overwhelm the system. But REL has great software for setup, so careful setup and you’ll be good.

Yeah I currently have the existing 9x set really low.  Don't take much and mud happens

Your stated main speaker response isn't digging very deep so the sub will have to reach up a bit to cover the mains.  This being the case, I would expect that a two sub setup will blend much better and produce fewer and less aggressive nodes.  If your mains were rock solid down to 35hz one sub covering the bottom octave 20-40hz might get it done but that's not your reality.  It would be ideal to get a dealer demo loaner to play with your existing sub to see how the pair works in your space.  Good luck and cheers

Get a second T/9x.  It does exactly what @ghdprentice says.  Soundstage extends in all directions and imaging improves.   I’m not aware of software for setting up a REL.  Neither of my T/5xes came with software, but they do have good videos on setup and integration.  My pair was relatively easy to get sorted.

what does the REL website speaker pairing suggest? If it says some s/510’s are the answer, then it’s a matter of getting the girl and then save for her twin sister. 👧 👧. 

A single sub can work great for a single listening position, otherwise lots of potential room problems...I bought a single larger REL, then another a year later...single sounded fine, but two was a very nice improvement...

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!!!

It's like boobs, two is best.

I had recently bought and added a Rel S510 to my system. The upgrade was so noticeable and uplifting, I added a second one a week later and never regretted. Everything works more in coherent and the soundstage is amazing.One is good, two is excellent.

I have one subwoofer and it took weeks to find that perfect spot, but with two subwoofers, it definitely makes positioning much easier.

"I have one subwoofer and it took weeks to find that perfect spot, but with two subwoofers, it definitely makes positioning much easier."

I have 4 subs in a distributed bass array and positioning almost doesn't matter.

I have S/510's.  The second sub is like @vicdior says, noticeable and uplifting.  Having 2 subs makes the positioning less critical.

I use the longbow wireless transmitters, it makes placing the subs even easier without wires.  

Definitely add a second sub.

If your goal to simply bolster your current presentation adding another -6dB Sub-Bass speaker should provide a noticeable difference.

REL, "subwoofers (I still like our original term Sub Bass System for its more complete description of what we do)"...

If you're interested in increasing the detail of the presentation as well as the addition of the delicate extra low fundamental frequencies found on many of todays modern recordings there are a number of economical -3dB subwoofer/s available. Positioning within your rooms standing wave bass mode/s, would be required for their optimum performance.

Actual subwoofers are adjusted, not matched, to integrate with any speaker choice.