Right now I am using an amplifier. However, in the past I have used the power from a head unit to run my front and back speakers. As others have already said, you really should also put in a subwoofer, which will require an external amplifier. If you go this route, I would also recommend some passive filters (inductors) on the front and back speakers to reduce the low frequency content to them. Obviously, these recommendations change depending on the volume you listen at and if you must have audiophile quality sound.
Also, I have never owned one but I have always been impressed with the Nakamichi head units.

Good Luck,

Alex