My significant other owns a Subaru Forester with a double DIN head unit that has a 6 cd changer built into the face. I don't know the exact year but I believe it is around '05 and it is one of the earlier models of the Forester that has the much "boxier" design. The stereo must have been the upgraded package as well because in the rear of the cabin there is a small subwoofer built into the side wall of the car (hardly noticeable sound-wise).

I have installed H/U's in Subaru's before using the stock speakers but have never dealt with one that had a sub built into the system. Would the sub still work with a new (Alpine) H/U? And if not, would there be any issues just leaving the sub disconnected and only using the in door speakers with the new H/U?