I'm pretty new to mixing/mastering and my question is as follows. When I'm listening to my song through my headphones or monitors, it sounds great, I feel like it's done or almost done...then I go ahead and plug it in to the aux cable in my car (which has a pretty good sound system) and it sounds horrible. All of the depth is gone and it's just so empty compared to how it previously sounded. Any clues as to what's causing this or what I can do to fix it?