The amp to sub connection has nothing to do with the HU's internal amplifier, but it will damage your sub amp if it shorts. without a meter it's hard to define any electrical problem, but try this if you can: Take a speaker that you know is good, and is 4 ohms. IE any coaxial speaker will work. Unhook all of the HU's speaker leads from your car and one at a time hook it to the speaker. If it works like that you're in luck, the unit is only protected. If not, it's possibly busted.

I lived out in Cross Creek when i first moved to FL 4 years ago, nice area!