The DIY option will, no question, get you a far better subwoofer IF the process is done correctly. But, this depends on the cabinet design and driver used.
I can specify a driver and amplifier and cabinet system that will give you superb results.
Driver: Infinity Kappa Perfect VQ 12" :
http://www.hookedontronics.com/show_product_details.jsp?cid=20835&brandtype=0
$165
Amp: O Audio 500 Watt Plate:
http://www.oaudio.com/500W_SUBAMP.html
$230
Cabinet: 3.9 ft^3 net internal volume
Port: Folded Slot Port: 3.25" H x 12" W x 56" Length
Use Mid-Q insert for Kappa Perfect VQ driver with above enclosure.
I can not stress this enough: there are extremely few drivers that will truly make excellent music subwoofers through a wide range of SPLs. Most drivers have motor and suspension systems that change properties radically as you increase power to them and increase excursion. The driver above, is by way of 3rd party verification, extremely linear. It will act very similar at both high and low power, resulting in extremely low distortion, even at full power of the O Audio amplifier. In fact, the ratings on that woofer are VERY conservative. It won't even be pushed near it's limit with the amp that is rated at 500 watts. Also, while rated at only 14mm xmax, that driver is absolutely linear within this range. Actual mechanical excursion limit(that is not linear) is well over 20mm one way.
The amplifier suggested is one of the very few plate amps suited for high power ported applictions because it has a variable subsonic filter. It is critical to use a subsonic filter for home theatre in vented high power systems. A high power peak that resides under the port tuning frequency for a sound effect could easily cause the driver to unload; this means it could push the driver past mechanical excursion limits and crash the voice coil former into the back of the motor assembly = dead driver. Besides having a subsonic variable filter, the amp is one of the higher quality units, and is made in Canada not China. It has a real 430 watts at 4 Ohms as measured by credible 3rd party.
The cabinet suggested is critical to use the vent suggested along with it. Most ported designs, both DIY and commercial have substantial acoustic output dropoff/compression near and at port tuning frequency. This means the port has insufficient cross sectional area to allow air to freely move; the velocity becomes too high causing non-linear air movement. The very substantial cross section I specified will insure you have full output with zero air noise even at full power with a steady state sine wave at 20Hz.
The suggested design above, in an average room volume of 1800-2000 cubic feet or less should easily produce in excess of 112dB from 20Hz on up with NO audible distortion using the O Audio amplifier. Output of 115-117dB would be easily possible with a more powerful amplifier. Avaserfi on this forum achieved around 120dB from 20Hz on up using a 2000 watt amplifier in an average sized room using a single of these subwoofers in the cabinet system suggested. This is NOT a SPL woofer, even though it can produce far higher output than most commercial subwoofer home systems. It is first and foremore a sound quality driver. To get substantially higher output witha single 12" driver you would need to use something along the lines of a JL Audio W7 12" driver in a proper slot port design.
I suggested the above cabinet. Avaserfi has posted a slightly revised version along with full cabinet cut list and box model for easy construction:
http://forums.audioholics.com/forums/showthread.php?t=45528
What commercial price range does this compare to in regards to performance, you might ask? Well, it's hard to say. This will result in a completely transparent/neutral subwoofer system with substantial dynamic ability if constructed as specified. Considering the cabinet and driver quality, if say sold by a make like Velodyne or Paradigm, it would probably cost in the range of $2500 USD. But I would expect they would use a more powerful amplifier to take advantage of the Perfect VQs full potential.
-Chris
