You can add as many amps and speakers as you want. You need a good high level input (such as the JL Audio cleansweep or audiocontrol LC2i to name a couple), so you can take the signal from the existing factory amp and then run it through to your new amps and speakers. Many people have added subs into the e70, me included, and it's not that bad. There are a couple of DIY's floating around that show you how to do this step by step so it shouldn't be too far out of one's comfort zone if you're somewhat mechanically inclined.