First, check for simple issues such as mute accidentally turned on. Some Kenwood models have a preamp-only mode that turns off the internal amplifiers. It's buried deep in the menu options. Check for that and disable if necessary. Also disable any demo (demonstration) mode in the menu since that can also affect the amp's output. It's possible the head unit's amp is bad, but that's unlikely and especially unlikely to affect all channels at once. It's more likely a menu setting or a wiring/connection problem.