In order to get better midbass response, you need to do some sound deadening in your doors. Aftermarket speakers can usually be installed with plastic rings to adjust the mounting depth. Leave the amplifier even if you want to remove the sub. Aftermarket speakers will run better off with an external amplifier than the headunits built in amplifier. If you space for speakers in the the rear doors, install some Aftermarket coaxils there too. Run all four speakers from the amp.
Remember, your speakers won't produce the same low end tones that a sub can. However, with some good after market coaxials, an amp and some decent sound deadening, you won't lack a punch