An otherwise elegant 5.1 sound system, the Vizio S4251W has a known bug which causes the subwoofer volume to jump to full volume from time to time, even with the latest firmware update.
You will know when this happens. If you move the subwoofer volume up or down, it returns to the previously selected setting. Not the end of the world, but since humans have largely uncontrollable physiological responses to loud unexpected noises, this intermittent characteristic is highly undesirable. While the exact conditions that recreate this bug remain a mystery, a fix is in using the Logitech Harmony Smart Remote/Hub, and you can probably use this sequence with other home automation solutions, too.
Having tried several different approaches to alleviate this problem using the Harmony Hub‘s Customize Activity options, here is an algorithm that works well.
For each Activity, add a variation of the following example sequence steps 4-12 just after the default sequence:
This causes the subwoofer volume to reset to a specific level, in this case level three, each time an Activity starts. With this configuration sequence, the loud subwoofer volume bug is no longer a problem. Without the initial VolumeUp/VolumeDown, the sub volume sequence didn’t seem to register reliably when switching Activities or coming out of low-power mode.