Noob question about upgrading stock Prius II sound

There's a sticky thread with all the instructions you need here:
2010 Toyota Prius model II Head Unit Upgrade | PriusChat

I think the upgrade which improves sound the most is to replace the 6x9 front door speakers. I bought some good-quality JBL's, but nothing super-expensive and esoteric. Bass and treble both improved very noticeably with this. Follow Spider-Man's instructions carefully. In particular:
1) Buy a Pop-rivet gun. A good-quality one.
2) Carefully read how Spiderman re-used the bracket of the stock speaker as a mounting bracket. He describes it for the rears, but it also applies for the fronts. So you do not need to buy a speaker mounting bracket.
3) Consider installing a little bit of sound deadening material, such as Dynamat. You don't need to go nuts and cover every square inch with the stuff. Even a little on the inner and outer sheet metal will deaden vibration substantially.

I next replaced the dash board "squawkers" with better speakers, but it's a lot more work and less improvement results. But you are replacing some complete garbage speakers, so there is gain.

The rear door speakers did not improve much. I think it's because I used 6 1/2" low-profile speakers. The cones don't move much. From the front, at least, I notice little quality from the rear speakers. Maybe different rear speakers, non-low-profile, would sound better, but I didn't test anything else for fit. I think they'd fit, though.