I really wouldn't recommend using a preamp booster app, as the N7 speakers have been known to easily blow out. The speakers aren't one of the strong points of the N7, though that's true of all tablets. In my opinion, they (or any tablet or phone speakers) really aren't intended for serious listening, but more for hearing notifications, game sounds, scanner radio apps, or basic YouTube or internet videos.
For serious listening of movies and music, use either a wired headset, a Bluetooth stereo headset, a wired, self-powered speaker system (what you were thinking), or a Bluetooth speaker system. I personally think that a headset is best with the intimate environment of the 7" screen of the N7.