As others said, adding another amp + subs is not a good idea. The reason is that you have a sub (designed to be in a small enclosure) firing into the ski pass, using the whole trunk as an enclosure. On top of this you are going to add two more subs in a box. This will most likely cause cancellation.
Keep the amp you have, ditch the L7 (which is highly inneficient), and get new subs + a properly designed enclosure.
No need to add a second battery (this will actually cause more strain to your car's electrical system than help). If you do have an old battery 5 years +, might not hurt to upgrade with a high-performance battery (optima, stinger, etc).
If you have already have a cap, won't hurt to put it in. It is not going to drain your electrical system as others suggested. If you don't have one, don't bother.