Originally Posted by ncx

With the LP filter in the off position, you will get whatever signal the factory amp was sending to the OEM subs which is supposed to be somewhere around 80hz and down. It sounded more like 100hz in my X5. If you turn it on, 12dB slope is fine, you will ensure than nothing much above your filter point will enter the amp's output section and on to the subwoofer. IMO, it is a good idea to have it on so you can keep potentially muddy upper-bass from entering your stealthbox. Good luck!