@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.