Sir get the Fusion Encounter FR693:
EN-FR693
or the Pioneer D6902R:
6X9? 2-Way Speaker 360W Max power handling - TS-D6902R :: Pioneer Australia
They are the best.
Your Panasonic stock HU is not the best and no stock player can power the rear 6 X 9 adequately because they have a limit of 12-28W RMS max while the rear 6 X 9 are from 40W RMS to 100W RMS typically.
However, good speakers will sound good without AMP at any power level.
Kenwood 718/EX are a size bigger and will not fit in.
The proper way is to change the front round speakers first since you will be listening to front ones directly:
Decent set of components like Sony GTX 1621 or Pioneer A1702C:
XS-GTX1621S : Speakers : Xplod Speakers / Subwoofer : Sony Middle East & Africa
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