Drivers:

This subwoofer was originally designed around the 18" Dayton Audio PA460 pro audio woofer.  An odd choice for a home theater subwoofer design you might say. True, but with the right balance of enclosure, power and proper EQ it does surprisingly well.  The PA460 is very efficient in the mid bass leading to very little thermal compression and great dynamics in that range. While it is a bit limited in terms of coil overhang (xmax) compared to more conventional subwoofer drivers it is an 18" driver so it has plenty of surface area to move air with.  It also has a very soft suspension and decent mechanical clearances so there is no trouble pushing it a bit at lower frequencies.   I have done thorough testing with the iNuke 3000 and the driver stays safe even when driven hard below tuning with the DSP settings limiting any bad noises from either the driver or the amp when being pushed hard into the limiter.

With the price increases on the Dayton PA460 the newer 18" GRS 18PT-8 looked to take over the value throne.  Indeed after testing the driver it performs almost as well as the PA460.  It does have some limitations such as less xmech which prevents use of the 15Hz tune and slightly less output and higher distortion in the low bass but at a fraction of the current price of the PA460 the GRS 18PT-8 is absolutely the current value champ.  Surprisingly the 18PT-8 has much lower distortion then the PA460 in the upper bass, so if you are looking specifically for the cleaner upper bass these designs provide over conventional subwoofers this may be the better driver for you.

Amplifiers:

The VBSS was originally  intended to be used in conjunction with the Behringer iNuke 1000DSP or 3000DSP amplifiers.  These amplifiers featured an onboard DSP which handled all of the DSP settings required to maximize the performance of the subwoofer.  Behinger replaced the inuke line with the NX series amplifiers, these are nearly identical spec wise to the inuke series worked with minimal tweaking of the DSP settings.  

The 1000 series placed in bridged mode can power one or two of the VBSS subwoofers cabinets.  

The 3000 series can power up to 8 standard VBSS cabinets, four per channel connected in parallel.  

The 6000 series can also be used however amplifier power is only maximized only when the drivers are used in sets of 2 placed in series then those sets connected to the amplifier in parallel.  This allows up to 16 drivers to be connected to one amplifier, 8 drivers per channel.    

Other amplifiers can be used if combined with a miniDSP or other similarly capable DSP to handle the required settings that gives these subwoofers the correct bass extension and protects them from over excursion.  The amplifiers used should be capable of 200-400w at 8 ohms for a single subwoofer per channel.  For two subs per channel it should be 4 ohm capable with roughly double those wattage specs into 4 Ohms and for 4 subs per channel it should be 2 ohm capable with roughly 4x those wattage numbers into 2 Ohms.  Some wiggle room on those wattage numbers is fine.

Standard VBSS Cabinet Design:

The standard enclosure is a fairly straightforward ported box, approximately 6.75 cubic feet net internal. Measures 23.5" wide, 31.5" tall x 20" deep or 20.5" deep if you add an additional 1/2" baffle layer to flush mount the driver. If going with a single baffle layer and surface mounting the enclosure can be cut from a single 4x8' 3/4" sheet of MDF or plywood. Add a 2x4' sheet of 1/2" if you want to flush mount the drivers and ports which is how I built mine.

Damping material technically isn't required however I still like to use it as i feel it can help clean up standing waves and ringing in the upper bass.  I used recycled denim insulation however that has been difficult to acquire for the past few years.  Acoustic foam or poly batting can be used instead, alternatively polyfil can also be used if something something is used to hold it in place.   

Ports:

The standard cabinet uses two of the 4" Precision Port Kits. 

For the 30 Hz tune the ports should be assembled without the center tube (flare to flare).

For the 20 Hz tune they should be assembled with the center tube at full length. The proximity of the internal flare to the rear cabinet wall helps to extend the effective length of the port allowing for the tune to be lower then would otherwise be achievable with the full 17" port length in this cabinet volume.  For this reason the cabinet depth should not be modified and placement of the port on either the front baffle or rear of the enclosure is required.

For the 15Hz tune it's the same as the 20Hz tune except one of the two ports should be plugged.

Alternate Cabinet Designs:

Dual Woofer Cabinet Design:

A dual woofer cabinet could be built with outer dimensions of 20" wide, 48" high and 28" deep.  An additional 1/2 or 3/4" baffle layer should be added if you want to flush mount the drivers.  This could be built using either three of the 4" precision port kits at full length, or using a slot port.  I did not design internal bracing for these cabinet designs but figure you want bracing every 8-12".

Panel Sizes:

2x 26.5" x 48" sides

2x 20" x 48" baffle/rear

2x 18.5 x 26.5" top/bottom

Measurements:

Comparison of the Dayton PA460 and the GRS 18PT-8 VBSS designs.

DIYSG 6cuft VBSS style cabinet used in 20hz configuration, single channel of inuke 3000 amplifier used. 

Measurements/SPL at 2m outdoors ground plane.

I started these tests on the PA460 at just an arbitrary level when I should have adjusted the output at the first sweep to something like 90dB.  At the time I was just looking to see if the 18PT-8 would be suitable, what DSP setting it required and how it compared  to the PA460.  I realized after I had swapped the driver over and been messing with it for a while that I should have taken the initial sweeps at a more standard output level.  However you can still get a good comparison between the two.

Much of the limitation at the low frequencies (near tune) appeared to come from the inuke 3000 amplifier running out of steam rather then a limitation of the driver.  However the GRS did bottom on the highest level sweep below the port tuning I ended up revising the highpass filter order from 12dB to 18dB/octave on that driver's DSP settings however the 12dB highpass is shown in these measurements.  

Notes:

The Dayton PA460 edges out the GRS 18PT-8 at low frequencies around and just above tune in terms of output, compression and distortion however the GRS has significantly lower distortion at frequencies above 50 Hz.   

DSP Settings:

DSP filter files for the Behringer inuke and NX amplifiers provided + regular filter settings for other external DSP's (like the miniDSP).

Note: The Behringer DSP settings are configured as Biamp-1 which allows you to use a single input on channel-A to run both outputs.  This is useful if the subs on both channels will be equidistant or if manually adjusting delays for time alignment of the two channels. If you wish to use separate inputs for those channels switch the amplifier mode to dual mono.