A little late in participating.
First my approach is not about what design of sub to use, I pretty much don;t care. I have not seen a big difference from the design or the maker, its all about the room and the setup. So I will de-emphasize the type and focus on the "how".
In my setup the room is special, its designed to be a listening room and it has extensive LF damping. This is always going to be an advantage. But the approach is the same no mater what kind of room you have. The idea is to use multple subs located around the room to smooth the spatial and spectral response at the seating location. The fact that multiple subs does this has been proven time and time again and really isn't an arguable point. IF you want the smoothest bass, then you must use multiple sources, no single source can compete. The room dominates the LF situation in any audio system and its the sources that must be accomodated to the room.
Sure its not great to have subs all over the place, but you have to get your priorities in order. Do you want the "best" bass or not, because if you do, you better get used to multiple subs. But rememebr that because there are lots of them, they don't have to be extremely large. Mine are about 18 x 16 x 14 for what I call the broad band subs which cover about 50 Hz - 150 Hz. These then overlap the mains, which are simple closed box. I use one Ultra Low Frequency sub that covers 25 Hz - 50 Hz for that lowest octave. At these frequencies only one sub is needed and it matters not at all where it is placed as long as its NOT in the middle of the room (given its size that would never happen.)
The most complicated part of using multiple subs is setup. To get it right you need some measurements to set the parameters on the sub amps correctly. This is done top get as smooth a response as possible. A detailed explaination of this technique can be found at DIY by Markus Mehkau and on his web site.
I am thinking about doing the setup calculations for multiple subs as a service to my customers, but this has to be worked out.
I use no EQ in my setup - although it could use some to improve it. The ideal setup would use something like a Behringer DCX9624, but I haven't bit the bullet and got one yet, because I'm not sure how much benefit it would be in my setup. As Randy has stated its pretty good the way it stands.
Finally all of my subs are very inexpensive, the plate amps about $100 and the B&C woofer about $150 makes up the bulk of the cost if you DIY, but the enclosures are pretty easy to build.
Thats all the time I have for now - later.