Well, as you can see he was using a Kicker comp series sub... pretty cheap, only like $80 I believe. Another sub thats on the same level, could be a MTX Thunder series sub (cheaper, I think...), or a JBL grand touring...
As I mentioned I saw that Kicker sub, however it is no longer available anywhere. I'd get the exact same model but it's nowhere to be found.
I'm trying to find a sub with the right specifications so it won't cause a problem. I can't seem to find any that say they'll work under RMS 40 or whatever it's supposed to be.