Boy was this an entertaining read.
So now that the discussion has wandered across the landscape and back, I'd like to remind people that DIY and production design is quite different. Modern, cheap DSP like the DCX-2496 and upcoming comparable products like the Woofer Widget will give DIYers much more flexibility and many more options than in the past. The fact remains that you will always have a hard time comparing DIY designs to production units. EQ and limiting which are more common in production designs make direct comparison impossible beyond limited frequency ranges and simple number comparisons. The differing strengths and weaknesses make for all sorts of interesting differences in percieved sound that won't be obvious nor easily quantifiable. While there are plenty of shoddy subwoofer designs in boutique stores, many have been carefully tested and optimized, with specific choices made in the final frequency response through both woofer parameters and on-board EQ. This is rarely part of DIY discussions, as few take their subs outdoors and measure the end result, but is rather assumed to be equal. I'd say that sometimes it is, sometimes it isn't. Of course others will note that EQ of the in-room response is all that matters, where I hold a somewhat different opinion.
There are plenty of areas I see DIY options being highly attractive, after all that's how I most of us in the business got started. Most obvious is the self-gratification of working on, designing, and constructing something that actually sounds good and impresses yourself and others. Beyond this aspect, there is no question that those who don't mind or can find a means to accomodate a much larger enclosure can do some serious damage on the cheap. This can range from a private missle-silo, to a closet sized subwoofer, and even to some creative construction to gain space in an aesthetically acceptable manner.
In the end the real advantage the DIYer has is the flexibility to maximize the size of the subwoofer by building to the specific application. On the flip side, most DIY designs are less tolerant of being over-driven, or parts will be used significantly below their maximum capability in amp power or excursion. The tools are now there with DSP, but I don't see many tweaking the limits. Of course actually testing the limits means you chance killing a few parts in the process.
That gets expensive for a DIYer, where it's required for most production offerings.
For the most part, if you aren't willing to go with a significantly larger enclosure, greater power (usually with a noisy fan), or are building to a required unique shape, the cost savings aren't always that big when you're looking at less than $1,000 spent given the power OEMs can now get from very economical plate amplifiers. As budget and available space increases though, you can most certainly do some things we just can't readily justify in a production product, as the interest would be low, or the logistics more pain than they are worth.
The debates of "what is better or equal" will circle endlessly. Some DIY executions are extremely high quality. Some miss some important details. The fact remains that unless you test your specific design carefully, and often in isolation, you are guessing that it did what you expect. This process of honest correlation to the plan is usually the difference between a good vs. an excellent result.
... and now back to your regularly scheduled drama.