I think where we can all agree is that there are a few ways to improve your sound system in one way or another. The general ways are..
1) Change the head unit to an aftermarket unit, OR, clean up and amp the signal via a Cleansweep like device. This is going to make the system sound from absolutely terrible (bad highs, bad mids, no/severely distorted lows), to above average (good highs, good mids, mediocre/artificial lows). It still wont be perfect without a Sub.
2) Change the speakers. This will give you the capability of getting excellent sound across the entire spectrum, as long as the source delivering the sound can send accurate information to them. On a stock system without a sub, depending on the speakers, the high's and mid's should be noticeably better than they were before, and the lows should be much better than before. It still wont be perfect without a change to the head unit (or cleansweep), and maybe a sub depending on the speakers.
3) Add a Sub. This gives you the ability to take everything below a certain level off of the speakers entirely (likely 70-80hz with the stock speakers i would bet). You will instantly notice that the speakers sound better since they don't have to try and do the lows. Highs should be decent, mids should be decent, and lows should be excellent. Still wont be perfect without a head unit change (but will be much closer)
The debate seems to be which one of these 3 gives you the most bang for your time/money. My vote is 1/3/2, and it sounds like Magnums (correct me if im wrong) is 3/2/1. Either way, throw some money at it and it will sound better