In reply to:

VTF stands for Variable Tuning Frequency. That model of subwoofer allows you to plug (max extension mode) or unplug (max output mode) one of its two ports to customize it's output to your liking.



Just want to point out that while other subs allow you to plug and unplug ports to tune the frequency, Hsu also has an EQ switch to optimize the electronics for how the ports are configured.