So Shep, I have a 2016 eco boost premium with the shaker system - and no trunk subwoofer. If I push the fade to rear speakers only, there is no basis whatsoever. I thought replacing the speakers would be the way to go, but I was wrong. My guess is the whole system is designed to work with a subwoofer in the trunk, and the rear deck speakers are just midrange/treble units. The head unit sends signals in frequency ranges, eg bass, midrange and treble in the front of the car, and I believe at the back also. So,,,,,, changing the deck speakers did nothing to improve the bass output, because the amp doesn't send any bass output to them. This makes sense for the system when it has a trunk subwoofer included, but if not, the rear deck speakers are pretty much a complete waste of time. I am not happy with my findings so far.
Sent from my iPad using Mustang Evolution