X2 on that pdx-5. 400 is a great budget for some good components just keep in mind you are going to want to go in this order.
1. Head unit
2. Amp and speakers at the same time
Here are a few recommendations. Keep in mind all these will have clear sound and all will sound a 10 times better than the bose system.
Focal Performance PS 165 6-3/4" component speaker system at Crutchfield.com
WoofersEtc.com - ES-620X484 - CDT Audio 6.5" Component Speaker System
WoofersEtc.com - H600S - Diamond Audio 6.5" HEX Series Component System w/ Silk dome Tweeters
WoofersEtc.com - CXS62 V2 - Image Dynamics 6.5" 2 Ohm Component System
Keep in mind a silk tweeter will usually give you a smoother tone where as a aluminum tweeter will be more bright.