The rw-12 and Jamo650 are definitely at the top of the budget list now that they can be had for much less than their original retail prices. Although 10 decibals at 20hz is a pretty noticable difference for the titanic DIY kit sub, even though it likely hasn't reached its potential in that box. 1k is a bit steep for a couple of DIY titanics even if they each ran their own amp. If you get the driver and amp for $350, a sheet of mdf is about $40, then the finish just depends on what you want. Another note is that in a properly designed box a single titanic or reference series sub models and has been measured at 100+ db at 20hz.
I guess my point is that a single properly designed DIY Dayton sub will outperform even the best budget subs in output and extension, and can be build for around $400 is you go with the reference hf and don't spend a fortune on the finish, yet you would have something capable of performing with subs that cost twice as much. If your budget is under $500 your obviously looking at the best bang for the buck. If done right the DIY route is the best bang for the buck.