Depends on what kind of music you want to hear or like hearing. That will tell us the type of Sub you might need. Also if you are adding a 2nd amp, better be a mono or a powerful 2-channel.
So do you want to go loud with the expense of cutting out on quality or do you prefer quality over loudness? And how much are you willing to pay?
Installing an additional amplifier to power up the sub will take more juice out of ur power generator and battery. So you might need to spend a little extra on thicker gauge wiring + preferably a BIG3/BIG2 upgrade.
So whats your battery and alternator Amp ratings?