I have a 2 channel bridgeable amp w/ high level inputs. Also have a low-profile 10" kicker sub and enclosure sitting in the closet. I am running the stock HU. It sounds like sh*t, as we know

Would the sound benefit more from amplifying the front speakers (stock) or adding a sub with a line out converter?

Adding an aftermarket HU is out of the question for now. I also have 6.5" Polk dB's sitting around, but don't know if its worth adding them with the stock tweeters..