What should the subsonic filter be at

The subsonic filter is to help your system's overall performance by blocking frequencies that your subwoofer cannot effectively produce. For example, if you have a single 10 inch sub, it's not likely to be able to produce sound much lower than 60-80Hz with any respectable volume. If you have the subsonic filter on and set to 60Hz, it will prevent a 40Hz tone from being delivered to the sub. If the subsonic filter is turned off, your sub will get everything, and it will try to produce every sound. If it gets a 40Hz tone, it will make the sub work extra hard without any audible noise. It'll just degrade the overall so the answer to "shoud the subsonic filter stay off" would be "it depends on your system" but in most cases, you should keep it on, and set it to the low end of your subwoofer or woofer range. This also applies to filters on mids and highs.