I recently finished a sub that some might find interesting:

2- Tumults

Opposite-firing, push/push, to reduce cabinet vibration.

Custom hourglass cylinder, 1/2" thick PVC shell.

1 1/4" thick solid, custom CNC machined Mic 6 aluminum baffles.

On board L/T circuit to flatten and extend frequency response.

Only 2 1/2 cubic feet net interior volume.

One 1,050 Watts into 2 ohms switching amplifier PER VOICE COIL (4,200 Watts total)

Adjustable system 'Q', from .45 to 1.2.

Variable phase, 12 position, in 15 degree increments, plus a 0/180 switch.

Gain, phase and Q controls on the front panel. RCA/XLR inputs and detachable power cord on the back panel.

Signal/clip/protect indicator LEDs.

Lighted front panel control knobs to indicate position from across the room, at a glance.

I've built 17 subs in the past 9 or so months and used 22 drivers from around the world. The Adire XBL2 motor, which is basically a multi-gap, underhung (Dan Wiggins told me he prefers 'inner hung') design, is definitely for real.

BTW, this is a dedicated, discrete LFE only sub. I have a similar 2X15, using custom made drivers for dedicated redirected bass that's optimized for integration with my sats, but it's only powered by 4-800 Watt amps.