1. Yes. You need more power to reproduce bass cleanly. You also need speakers that can handle that type of frequency range properly. Which is why people run speakers at home with 10-15" woofers, or separate, amp'ed, subwoofers.
2. Head units are generally: AM/FM receiver, CD Player (usually with MP3/AAC/WMA capabilities [which is just compressed audio files burned on a data CD]), Aux-Input (could be USB, or just regular line-level input). Most have amplified outputs, with standard color-coded speaker wires, and they generally have line-level output. The output voltage of the line-level is anywhere between 1V and 5V, the higher the better. Ya know, the whole signal to noise ratio thing. If you install an amp attached to the line-out of the head unit, the amp must be on in order to hear anything.
3. You need to look at the CEA power ratings (http://www.ce.org/standards/Standard...*.aspx?Id=1455
) to get some accurate numbers. My Kenwood amps are all rated by this method. Of course, signal to noise ratio and THD% are important, too.
4. Depends on the head unit. My Kenwood KRC-X790 has 3 sets of line level outputs. Front/Rear/Sub. The front/rear are controlled with the balance/fader controls, and have a high-pass crossover built in. The sub outputs are controlled with a separate "subwoofer" level, and have a low-pass crossover built in. So, I have 6 separate RCA lines (3 pairs) going to the two amps under my back seat. (One 4-channel for the door/dash speakers, One 1-channel Class-D for the two 15" subs.)
5. Only if you want to be able to adjust any controls on them. Mine fit nicely under the rear seat, and still have enough air gaps to breathe fairly well. I lift up the seats if I plan on stressing them past normal listening volumes.