Sorry for the late reply Sir.
Thank you for liking my setup. Bro almost every head unit does have a built-in amplifier. But they are not that powerful (only 15 watts RMS approx) yup they are enough to run the stock speakers but since you have speakers which require alot more power to bring the full potential out of them, so adding an after market amplifier would be a very nice investment.
About Panasonic pre vs Pioneer pre: Well i guess the very 1st thing you should invest on is the amplifier, later on if you feel the need you can change the head unit aswell, but 1st buy a decent amplifier.
Hope this helps