Unknown to many, the secret to Dr. Hsu's exotic subwoofers had little to do with their external form, and more to do with what was inside: carefully engineered and matched components. Most engineers did not understand how to properly integrate all the parts in a subwoofer, such as the cabinet, driver, and amplifier. The idea came along in the late 90s of taking Dr. Hsu’s unique synthesis of components, and putting them into a smaller box—with an additional twist. The VTF series was born. Dr. Hsu's variable tuning technology (VTF) allowed for a subwoofer to play either low or loud by building into the cabinet two large, closable openings called ports. The "maximum output" mode became popular among home theater enthusiasts, who could fill large rooms with very loud bass by opening both ports. The "‘maximum extension" mode was preferred by music aficionados who appreciated pure deep bass by opening one port. Once again, imitation was inevitable. Today, the VTF series incorporates even more flexibility, allowing for sealed operating modes and a Q control for adjusting the bass tightness to one's taste.