Danteg

From my experience, the peerless XXLS is the best sub i have owned in term of sq. I'm using one 10'' right now in a 0.8cu ft sealed box powered by and arc audio amp (around 375w at 4 ohm) and to my taste it doesn't even come close to the JL 10W6 that i had for a couple of months ! The XXLS play just as loud as the W6 if you want him to shake your car, but the best way to enjoy this sub is at low to moderate volume where he will be very transparent. I don't think you can go wrong with a 10'' or 12'' Peerless sub !

mommjomma

You cannot make junk sound like jewels regardless of the box you put it in. That isnt to say that the box isnt important, its critical. Having said that, and with the assumption that you will be desinging and building the box properly, some of the best sounding woofers i have ever used personally and heard are the AVI SLS300(top end of your proposed budget) for sealed applications, and my new favorite woofer for the money hands down is the Focal PS seriers 12" and can be had for under 200 bucks

stochastic

Take a look at the subwoofers designed by Creative Sound Solutions http://www.creativesound.ca They're great bang for the buck in SQ with very low harmonic distortion. They also make passive radiators if you decide to add those to your box design.