Little help with REL subwoofers, please?

Thanks, @audiorusty. I'm sure you're right about the room; I could never tame that JL, even with an external crossover, some room treatments, and all kinds of fussing. Boomy, as you say, and sluggish sounding.

I've read that multiple subwoofers don't need to match each other, but would those small REL Tzeros help "balance" something bigger like the T/9? 

I like my current sound ... I just want deeper bass, not "more" bass. I may just buy a T/9 or T/7 and add it to the mix. We'll see! 

I've tried a number of subs over the years, even with dsp could never get coherent sound with main speakers. With dsp the problem wasn't so much with boom, but rather timing with main speakers, always a step behind or slow, once heard could never be unheard.

So, buying into Rel philosophy I recently purchased two T9X, last night first listening session with them. So, first impressions, first time I've heard coherence from subs with main speaker! Improvement in sense of sound stage ambience most noticeable change. bass extension more subtle with my Klipschorns. Perhaps it was recordings I played last night, but there was no earth shaking, wall rattling bass, just a bit of extension and fullness on bottom end, really integrated well with my initial settings on crossover and level. This bass performance quite different from my Wyred 4 Sound Genesis clone sub, which has the more spectacular, impactful bass sound.

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.