I don't think you folks are thinking this through.
There is little point in going through the hassle of fixing the carpet and not upgrading the sub while you're there. Probably best to add some amplification for that sub in the spirit of doing the job right the first time. Having done that you will have increased the weight back there enough that for the safety of your family you will have no choice but to install new wheels and tires.
On the other hand maybe i just don't see it the same way as others do....