My buddy has the factory nav system as I had in my f-150 and when you cranked it up, it would pull back power as if to keep the speakers from blowing. I now have a premium sound 2011 f-250 (no nav) with the factory sub and it goes way louder without hesitation. I also test drove a 2012 with no nav and it sounded awesome too. Kinda sux. My buddy with the factory nav ended up doing a full custom speaker system which rocks but cost him 4k. I think the highs and mids on the premium sound are good enough just to add a sub. Anyone else notice this?