tweeters use crossovers and share the channel with the front speakers. You need a 4-channel amp, not a 5 channel, or 4.1, those are pricier because they account for high wattage going to a sub.
I currently have my amp in my car plugged in to an ipod until my PC is finished and it is sufficient for my needs. I cannot max the volume without being uncomfortably loud. The android puts out about the same volume. If you like your stuff loud, use a preamp. If you like your music at more normal levels, you won't need one. Just be sure to get well-shielded audio lines. Ground loop noise is horrible if you skimp on the shielding/bonding for this.
Note: My amp has a sub channel and Alpine amps are pricey, but superb quality. The MRX-F35 would be what you need (if you go with Alpine) because it is a 4-ch amp and should provide plenty of wattage for regular speakers. The entire X-Power series is fantastic. If you are looking for something more akin to stock power levels, try to avoid going below anything less than 40x4 watts (160 watts, 4 channels).