A long time ago, I replaced my stock radio with a Pioneer P3600, which is a decent HU. The problem is that my stock radio had WAY more bass than my new HU has. Even with all the bass settings turned all the way up there is hardly any bass. I have to turn the loud setting on high to get bass, but then it sounds like crap and gets all distorted. I have Pioneer REV series 6.5s in the front and some regular Pioneer 3 way 6x9s in the back. Do I have to amp all the speakers to get my bass back or what?

thats totaly up to you if you wanta run an amp to them but chances are is that your stock hu put out more power then your pioneer does. if you wanta run an amp find like a 400 watt amp (100x4) that would work perfect for the speakers you have.