Best budget 12 inch sub to pair with 340W x1 (RMS) @ 4ohm

[jluv]

You should be fine with a sealed box...

If you want more output, and have the room, go ported!

[Juiced4v]

12W0v2-4 ported.
http://mobile.jlaudio.com/products_e...13&prod_id=592

This one! EW beat me to it! Good woofers! Cheap price

[ThreeFingerPete]

why 12's, 10's can hit just as hard with less power

A comparable 10 will not hit harder than a 12 with the same amount of power, given similarly(optimally) designed enclosures.