If you think the rears sound bad, they're too loud. Rears are for fill. Their tonality, in anything other than a 5.1 setup, is irrelvant. Also, I'll bey my ass that you have some phase issues. In 20 years of doing car audio I've never seen, built, or heard, a car without a processor with a delay, that didn't need at least one mid or midbass polarity reversed.
If you're going to add an EQ just skip and go right to a full-bore processor like a Zapco DSP-6SL or Audison Bit One if you want to do the tweaking yourself. If not, try the JBL MS-8.