it could be speakers or the stock HU. you can always pull the stock HU and test your speakers with a digital multi-meter. your speakers should all read 4 Ohms. if the speakers test good, it's proabably the HU.
what's your model year? are you under warranty? if not, this may be a good chance to upgrade.
what's your model year? are you under warranty? if not, this may be a good chance to upgrade.