Experience with stacked subwoofers?

Well, tomorrow is my day to try dialing in my REL S/510 four-pack. I’ve watched John Hunter’s Youtube about doing that setup for a six-pack. When my dealer delivers my last pair they will send their expert on setup to help get me sorted, so I’ll see how close I get to what they think is good, at least up to the point of working on fine tuning. I even have the Sneakers Track 4 audio they recommend , ready to go on Qobuz.

@simonmoon: The GR Research/Rythmik Audio Open Baffle/Dipole Servo-Feedback Woofer is definitely the best sub I’ve ever heard. The one in your picture is not actually a stack, it is the 2-woofer version of the sub, also available in a 3-woofer version (the dipole cancellation inherent in open baffle subs makes a single 12" woofer per side insufficient in maximum SPL potential). Most audiophiles will never hear it, as it is sold as a DIY kit. However, GR Research has a couple of cabinet makers who will build a pair for you (for a price, of course).

Advice for potential buyers of the sub: The Rythmik Audio A370 plate amp that comes with the kit is solid state, so is of course optimized for a 4 ohm load. The woofer used in the sub was optimized for open baffle use by Danny Richie, and is available in both 8 ohm and 16 ohm versions. Danny routinely sends the 16 ohm version with the kit, so that 3 woofers may be used with the A370 plate amp (the combined impedance of 3-16 ohm woofers is around 4 ohms). However, if you intend to go with the 2 woofer version, get the 8 ohm woofers rather than the 16 ohm. The 4 ohm combined impedance of 2-8 ohm woofers maximizes the power output of the A370 plate amp.

Having experimented with different subs and arrangements in-room  I can say with some authority that the results you get may surprise you. And it can go either way. My experience supports the fact that if a single sub is good, then two is way better, and not stacked on top of each other in one corner. While that gives you more output, it won't help control room modes and give you even bass response thru the room. Separate the two subs and treat them like they are stereo because bass is not monophonic. It does not mean they have to mirror image in terms of positioning but try running them on either side of the screen or tv out from corner along the front wall assymetrically and run some measurements. Keep playing with positioning until you are happy with the results and then do some DSP on the remaining peaks and you should be in pretty good shape. That second sub will even out the response and give you 3 db more sub output, helping perhaps cut down on low freq distortion a bit. Well worthwhile expense to add the second sub. 

@emergingsoul 

Stacking subs is not a gimmick or a marketing ploy.  But its effectiveness does relate to your goal sad others have said.  One sub is nearly Omni in a room and expose standing waves everywhere.  Moving it often just moves the stadium g wave/ null location.  If it’s for home cinema, most of the sub output is sound effects so almost everything relates to the screen.  If it’s for music, distributed arrays can work better using multiple sources turned down at lower levels  especially in ATMOS/ immersive.  I think Duke has spoken about this extensively. 
brad

@lonemountain 

Standing wave problem is overrated and most rooms don’t have this problem.  Lower bass waves are very long and permeate throughout the room quite well.

Also if you have decent main speakers for two stereo listening you shouldn't really need to have a subwoofer.  Fully endorse subwoofer or two for home theater, you want tanks to rumble your entire house when they move across the screen