Is the stock radio (in an 07 pkg 2) too weak to power component speakers?

Recently I had installed (in my 2007 package 2) a set of component speakers in both the front and rear doors. The speakers are 6 3/4" pioneer premiers with 1" tweeters. Before I purchased them the rep had played them for me on an in-store deck and they sounded great. I asked him if they were being powered by an amp and he said no. So, i figured they would sound fine in the car. Well, they don't!

The tweeters sound fine, but I'm getting no push in the speaker. There's sound and some bass, but nothing near what they sounded like in the store. Hell, the stock speakers had more bass. So, my question is...is the prius' stock stereo too weak to power these new speakers? If so, what amp should I have installed? This is my wife's car and she folds the rear seats a lot, so I'm looking for an amp which will store safely in either the cubbies near the rear lift gate or under the passenger/driver seat. Any suggestions on a good amp which will both power the new speakers and be small enough to store in these locations?

And if it's not the stereo, what's the problem? Why no bass? Since they're component speakers could it be an issue with the way the crossovers were set up?

Any help you guys can provide would be EXTREMELY appreciated. Thanks. Happy Prius-ing