As for an EQ going in, you can't use a passive because they are designed to be fed into amps and then to speakers next. I definitly would not recommend an EQ with a "booster" on it either. They were pretty much junk back in the day and still are now. It was funny how they used to say 200 watts on them way back when they were closer to MAYBE 25-30 watts.
To be about as happy as you can be without changing out all of your speakers, I would recommend getting a newer head unit with a built in EQ. Mine has one although I don't use it very much because I like a more flat and true sound rather than an altered sound. The deck I am using is the Kenwood KDC-679 Excelon. I think it retails for around $340 or so. Sweet deck, comes with a free Sirius satellite tuner as well. Then you can contact me and I can give you the info on how to splice the wires correct.
That comes from 16 years install experience and owning an install business now.