I have just aquired a 97 Expedition that is not equipped with a factory sub and i have no bass in the audio system but everything else is working just fine. my question is the bass controled by the head unit or by the underdash amp >. I do not wish to go to a aftermarket system . I would rather just replace the head unit or the under dash unit with a salvage yard replacement . just need to know wich one dictates the bass.

 

Bass will be dictated by both. The headunit will send the bass signal to the amp which will then send both the signal and the power to the sub.