can i use plywood for my sub box?

MDF is cheap enough to buy anyways....might as well drop the 20 dollars and get it.....  As for mounting the amp to the box.....if you have thick enough carpeting on the box, that should absorb enough vibration.  But the problem with plywood would be flex.  Even with 2 layers, its easy to flex plywood.  Without bracing, you could do damage to the box, not so much the amp, when the sub hits (in sealed boxes).  The air inside the box will be compressed with the woofer protruding, causing the plywood to flex in slightly.  overtime, it's possible for the box to crack, since the screws holding hte amp in will create slightly larger holes.................well its hard to explain, and i probably sound like a fool trying to explain...........its all physics...... with the pressure on a plywood box, and the amp mounted to it, trying to fight the pressure, you'll inevitably break somethingMDF is much more difficult to flex, and is a lot stronger.  Carpet it, and mount the amp right on top, using ALL screw holes on the amp, and you should be fine.  Also, if you're worried about hte amp, I know most now come with grommets right in the screwholes to absorb vibration.   If yours doesn't, you can pick a pack of grommets of assorted sizes at an Auto Supply store *like the Autozone* for a few bucks.  That will help with vibration.....As will some rubber washers underneath the amp's mounting holes.....***REMEMBER TO PRE-DRILL YOUR MDF!!!   IT IS VERY EASY TO ~wayland