I don't think running 8 speakers with one amp would work. And since you already have an amp, you'd need to start thinking about adding a battery if you add more amps. Ideally, you'd add a second battery to run your stereo components, and grab two Precision Power 900.4 amps to run the 8 speakers.
If you don't want to add a second battery, then forget about amplifying the cabin speakers and focus on the tower speakers and the sub. In which case, you could get away with a Precision Power 900.5 (5 channel), run the 4 tower speakers and the sub from the 5th channel, and get rid of your existing amp. That would probably be the only scenario where you wouldn't need to add another battery.