Try putting 6.5" speakers in the doors, and amping the speakers.

I put in 6.5" coax's in the front doors and an Alpine HU in the dash for $300 and I have a pretty nice sounding system to my ears. Maybe a little boomy/weak on the low end, and the soundstage isn't perfect, but that's pretty much it. If I had a basslink in the trunk that would pretty much be all I'd ever want to do.