I dident think subs weighed that much to make a difference.
I had a 12" sub and box in the boot of my Audi A3. I actually took it out cuz it became a pain in the backside. The box would slide from side to side and it took up too much room. The boot on the A3 isn't exactly the biggest. I can honestly say the weight of it didn't really come in to the equation.
If you are that worried about the weight then you really should look at why you have that car. The best ways to reduce excess weight is remove the spare wheel, jack, brace, rear seats, dashboard, door cards, sound deadening, glass windows(replace with poly sheets), drive with only five pound of fuel in the tank at any given time, drive naked and make you visit the toilet before every journey, travel alone(won't be hard considering your are now naked, in a noisy, cold car), oh yeah and finally make sure you empty the ashtray.
Or another option would be to upgrade your suspension.