When running speakers in parallel, the impedence is reduced. In your case, 8 ohms to 4 ohms. So the amp wattage really doesn't matter as much as it's ability to drive a consistant 4 ohm load. You'll need a good quality amp.
A pre-amp is basically a processor...you hook all your components (CD, DVD, TV, phono, cassette, 8-track, etc.) into the pre-amp. You then select the source thru the pre-amp. It then sends the signal (unamplified) to the amps, which are connected to your speakers. An "integrated" amp has both the pre-amp and amp in the same unit. These are the most common. Around here, you will find people use integrated amps from Harmon-Kardon, Denon, Pioneer, Yamaha, etc.
No...in order to connect another amplifier, you would have to use the pre-amp outputs on amp 1. This signal carries no usable wattage. The signal is then amplified by amp 2.
