So I currently have a 2008 Mazda 3 GT with Bose Sound System. It's not that I think its that bad at all, considering that when I bought the car the past owners never listened to music at all, or if they did it wasn't loud. Anyways I currently have a Rockford Fosgate P3 with a Kenwood KAC-5001PS 1000-Watt amp and its pretty loud and can blare out the speakers if wanted, and people in back of car can't hear the speakers much and speakers are starting to bottom out a higher volumes and I know new speakers with an amp would sound amazing. So with that being said I'm looking for 6.5" and 6x8 Speaker recommendations, I have about $300-$400 to spend for front and back speakers, and about $100 for the amp, probably getting a used amp from a friend or off craigslist/offerup

 

stick to 6.5 components. tweeter midwoofer and crossover.
for your 300-400 budget id go hertz hsk165 and call it a day