1. 20 watts to 100 watts. That more or less means 20 watts is the minimum for the speakers to function properly with most music. 100 watts means that if you exceed that and have the volume turned up considerably, you're in danger of blowing the speakers. However, you will find that more power is usually better power, as long as you realize the limits and don't rack the volume knob all the time.
2. No amp will take the signal directly from a CD player, tuner, tape deck or DVD player. Those components adhere to the "line-level" standard for output, meaning a pre-amp is necessary before you connect to an amplifier. In classic high-level audiophile systems, the pre-amp serves as the front end - with volume control, source switching, tone controls, etc. The amplifier is a separate box that does nothing but amplify the signal from a preamp (line-level standard input) and powers the load, most typically speakers.
Receivers combine the functions of both in a seamless way. Better receivers, however, allow you to separate the connection in back between pre-amp and amplifier within the receiver.
I'm not up on the current DAPs in terms of what they need. I suppose an iPod's output can be input directly into a separate amplifier, but you probably need to be very careful where the volume is set on the iPod, or it will send the amp into clipping. That can be dangerous to your speakers even if the power output is not up to that 100W limit. Better to use a "line-out" feature and keep things in the component line-level standards. That way, you know what you're buying and what it will do for your speakers.
