From what I have heard it is over priced and low in quality. You don't have to change your stock head unit to get a sub (but it's a good idea to so that you have more control over your audio levels). But all you need to do is go to a car stereo shop, hell, best buy might sell them, but there is a little conversion plug to allow aftermarket amps to connect to stock head units. I did that for my scion's sound system. I had a memphis amp with two 10" memphis subs.
What year is the subaru that you are working on?