a lightweight box means a smaller box.

my old setup use to weigh about 70 lbs. 2 12inch W3s in a sealed .75" MDF box. about 1.25cf per sub. IT WAS HEAVY!

if you want something ligthweight, i would go for a 10inch that requires little air space in a sealed box. the only one i can think of is a IDQ.

there is really no problem with using the stock HU and connecting an amp. of course there are drawbacks, but most people cant tell the difference.