Little help with REL subwoofers, please?

You can try a T9x or a T7x to see what happens. Hopefully you can get them on a trial basis. As long as neither sub can reproduce the offending frequency of the room with enough power you should be good to go, but if either of the subs can reproduce the offending frequency (standing wave) you will still have the same issue with boomy bass.

In a multi sub system, the subs do not need to be exactly the same but it is best if they all have a similar range. For example; lets say that one of your rooms standing waves is 29 Hz. if you have three or more subs placed around your room reproducing that 29 Hz. frequency, those wave forms will interfere with each other in such a way that the spots in the room where that 29 Hz. was loud will now be softer and where it was too quiet should now be louder, thus smoothing out the 29 Hz. peak throughout the room, making the room less boomy. IOW if one or more of the subs in your system cannot reproduce the offending frequency they cannot contribute to the solution.

Here is an article about room acoustics and boomy bass you may find helpful.

https://www.audioholics.com/room-acoustics/listening-room-acoustics-1#_ftn2

Forget about specs. They are severely misleading. As you note deep bass is not about hearing, it is about feeling. In this regard there is no substitute for surface area. The minimum in most rooms is two 12" subs to get solidly down to 20 Hz. I want some usable output at 15 Hz Consequently, I use 8 12" drivers. Large venues breath at very low frequencies. There is also signal down that low in movies. So, live music recordings and movies require the same capabilities. One sub alone is never a good solution. Two 12" drivers, Four 12" or two 15", eight 12" or Four 15". The last group is the largest required in the usual residential situation. 2 way crossovers are mandatory, digital bass management with delay capability and room control will give the best performance. 

Thank you all. 

@sns: right on! I hope you do a follow-up post somewhere out here on your experience with the two T9s. 

@audiorusty, thank you--that may be the most accessible explanation of standing waves, and the link was very helpful. I especially appreciate your explanation of why having multiple subs with different ranges may not work as seamlessly as I had hoped.

@mijostyn, wow--I can only imagine the bass you're getting with that approach. I appreciate that you're thinking about live recordings and recreating the effect of large venues. I'm an old Deadhead and I can remember some "Phil bombs" that were unnerving in their effect. 

@northman      Which model JL Audio subwoofer did you use and how did you determine its exact position in your room? 

Not sure if the OP just ordered the T9's or not but I have tried them all in pairs and I was shocked at how much more I preferred the T5's over any of them simply because of their active down-firing and for even less money than a single T9 you can have a pair of 5's. Regardless of the stats claims, I preferred the T5's because the down-firing was just more impactful and far more effortless while the others had to work harder to produce bass impact. I just could never feel the same bass with the others. For pipe organs I would not expect the T zeros to deliver. There doesn't seem to be mention of floor material. Since my floors are hardwood, they generate their own resonance in a good way so I went with the Tzeros instead of the T5's. Despite claims of 12" subs, they are too slow for great musicality and on the rare occasions of pipe organ they may deliver but for overall music, there's just no need for all that in my experiences with music. If people are into thunderous bass for movies and stuff, I get it but for pure musicality, forget it. We have the Magnepan 1.7i's and LRS plus.