What would I do, it would depend on my uses. Is this going in my living room, or behind the screen in my theater? Is this movies or music? How soon do I need it? How big my your room? What's my DIY skill level?
If it's going to be in a living room and needs to look attractive (or at least pass the WAF), If I could be patient, I would wait for a sale on the Monolith THX 10", If I couldn't be patient, I would probably go with the Speedwoofer 10s (which you already ruled out), HSU VTF-1 MK3, or if $400 was pushing it, a BIC PL 200 II. The BIC doesn't have as much output but looks better and is cheaper then the Hsu.
If this was going behind my screen wall in my theater, I would definitely go DIY and do the Dayton sub. I would probably do a stand alone amp rather than the plate amp I listed, but budget was a limiting factor. (For a plate amp I might even go with the Dayton 500w rather then the one I listed).
I'm actually not far off as while I have a more expensive sub upstairs, I have a NXG (now defunct) sub in my Theater that is fairly similar to the BIC. If you look in the "under $300 subwoofer" that's a sticky, there are some pretty good reviews on it. I am planning on building two 18" Dayton's in the future though and just painting them black. Since they will be behind the screen, they don't have to be perfect looking.
There are some pretty decent subs around the $500 level including the Monolith 10, SVS PB1000, SVS SB1000, Rythmic L12 but $400 is a much more challenging price point, especially if the Speedwoofer is off the list.