Would it be a bad idea to wire a sub to the stock head unit. Or should I get an aftermarket headunit.

 

There is no problem adding a LOC to run a sub off the stock system. Do know that the stock HU has a built in EQ that deteriorates sound quality with real speakers. So if you care about sound quality and plan on expanding your system, I would look into going with an aftermarket HU or getting a JL cleansweep or similar. I checked my stock HU after a month or so of having the car.