Unless you were trying to hear all the notes in a symphony, the stereo is fine. The bronco was not designed for audiophiles Given the amount of other noises that come through the cabin. That being said, 99% of what i listen to in the car is kid’s music and podcasts.
Are you sure you didn't have a back ache before driving the 100miles? If the seats had nails in them i would not have noticed in the first 100 miles. I had an 15 m235 and those seats became agony after 100 continuous driving.