50wx4 is the peak power, rms is about 20w x 4.
Putting 6x9's in the rear shelf will help a bit with the bass... but there wont be any real deep bass without a subwoofer. And besides, 6x9's dont, or cant adequately produce subbass, only midbass - midbass belongs up front.
Fit the largest possible speaker you can in the front, the bigger the driver, the more midbass it can handle. Midbass is what gives that punchy bass sound you're probably after.
Clarity, well, thats really up to the quality of the speaker you put in, and the install job. The headunit is perfectly fine. Have a check of the EQ on the headunit, and see if the settings have been bumped up or down.
I mean, have a fiddle with the EQ, or flip around the presets, even the "loud" function, and see if you can improve anything. Just be careful with the volume knob though, especially with the "loud" function.