The variable tuning option allows the SVS owner to adjust how deep the subwoofer will play.
The standard/native operating mode is 20 Hz with all ports open. The 20 Hz mode provides the most muscular and powerful presentation over the most common bass frequencies, and will provide usable in-room extension to 17-18 Hz.
If a single port is plugged and the amplifier Subwoofer Tune is adjusted to 16 Hz, the subwoofer will provide usable in-room extension to 13-14 Hz. There is a slight loss of maximum output capability over the more common bass ranges when the deeper 16 Hz mode is selected, but many enthusiasts find this trade-off well worth it for digging deep into the infrasonic region on DVDs and certain music tracks (like acoustic pipe organ).
If all ports are plugged and the amplifier Subwoofer Tune is adjusted to Sealed, the subwoofer will start to roll-off around ~32 Hz and will have a broad/shallow roll-off profile. This shallower roll-off profile is often an excellent match to the acoustics of a smaller room which has a lot of ‘room gain’ (i.e., a rising low-end response), and Sealed mode in a smaller room can often exhibit usable in-room extension to 15 Hz or deeper!
The bottom line is that the SVS variable tuning options offer the enthusiast the ultimate in tuning and deep extension flexibility to meet the needs of a wide variety of room sizes, playback levels and listening applications.