Velodyne HGS Series 1 vs Series 2

Anybody out there "in the know" on what the difference(s) is/are in the Velodyne HGS "Series 1" subwoofers and the new "Series II"? My theater room has an HGS-12 "series 1" sub, and I was curious what has been changed in the latest generation, and how the change(s) affect performance.

Thanks in advance.