Experience with stacked subwoofers?

Everyone hears differently and has different values regarding audio purchases. I've heard several systems with REL 6-packs. I didn't like the sound of any of the systems I've listened to with 6-packs. I actually own dual REL subs, so I'm not being a REL hater.

Thanks for all these responses. I have two Rel t7s in stereo in my current set up, which were one of the greatest improvements made. In my own personal experience, I have found subwoofer replacement and even toe-in to make a significant impact on the quality of the base that I hear from the listening position. Not just the amount of bass, but the ability to hear distinct differences in the notes from one to the next. Having experienced a dramatic difference with that, I’m imagining that adding a height dimension would also be significant. So I’m intrigued with the idea of stacking three subwoofers, each tuned to accomplish something slightly different. 

A good friend of mine has a pair of GR Research, Double Trouble open baffle subs. 

One stack per channel.

Probably the best sounding subs I’ve heard.

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.