system in a Jeep Wrangler

Hello. Noob here with a few questions about putting a system in a Wrangler. I am trying to keep it looking as stock as possibel 'cuz its a Wrangler and security is an issue. I replaced the factory head unit with an Alpine rated at 18 w rms x 4. I will be installing 4 Polk db525s. I would like to power them with a decent 50 x 4. The factory sub is 6.5", and it would be cool to keep it in its enclosure. I know a JL 6.5" will fit, and the JL 6W0-8 at 8 ohms can handle 150w x1.

Questions:

Should I power the Polks off an amp, and not the headunit?

Will a 6.5" sub give decent quality bass (less boom/more music)?

Would a 5 channel amp like an Alpine MRV-F450 be a good choice to power everything?

I have no backseat, and I would like to put the amp(s) under the front seats. Space is REALLY limited. Does anyone make a small decent amp for a good price other than Alpine?

Thanks

Bought some speaker adapters here:

http://www.mjmautoinnovations.com/sakj46t525.htm

The overhead pods are also 5 1/4".

You can just screw them down under the seats.  Double check both sides before putting any screws through the floor- to make sure you won't hit anything (gas tank, brake lines, etc.).  Also, be sure you have sufficient space for the amps to breathe properly/dissipate heat.