2000 stereo questions

ok, so i have searched the forums, and have come up with a little info regarding the stereo system in my wifes 2000, ie, "lexus" sound system and such. she currently has some after market head unit, aftermarket speakers in rear, stock in front, sounds like a tin can.

i have a nice sony head unit sitting around.(52w X 4 ). wondering about this "speaker amp" that is somewhere in this jeep, do i hook the speaker wire up as usual? and, suggestions for good speakers, front and rear. i dont want to "thump" the neighbors house, just have a more of a full sounding system.

thanks

Probably the wrong forum, but that's up to the admins.
There is no amplifier for the 4 speaker system in the XJ. Amp was only installed in the six speaker package. It's under the rear seat on the drivers side.
Amplification comes from the receiver itself, and the tinny quality is directly attributable to the quality of the speakers. The harmonic distortion and fidelity of the receiver does contibute, but no where as noticable as the speakers themselves.
Upgrade the speakers.