Downward firing subwoofer

I would just replace the sub, sell old one if you can. Just don't buy another downward firing sub, I don't like them.

It would be a problem due to the impedance load on the amplifier in the sub as well as mess up the sound of the bass from the subwoofer. If it is a passive unit with no built in amplifier then you would just mess up the tuning, double active driver subs usually have a larger box than a single driver sub.

Just try it and see what happens.

then buy a puppy.