There is an issue possibly with the specific subwoofer and the implementation. OEM often used Kenwood and Kenwood Excelon subwoofers in 'free air' applications. Kenwood, to my knowledge, has never actually made a free air sub other than the white dedicated marine sub. Some of the factory systems had the sub mounted in the kick panel facade which was completely open over the top. So even a true free air sub wouldn't have a chance to perform. If it was an automotive air suspension sub used in a free air application then it was underdamped and it was in fact bottoming out. That type of sub is dependent on the rigid air spring of a small sealed enclosure to control the cone motion and protect it from excessive excursion.