Sometimes high level connection is the only possibility, but it prohibits a high pass filter. So in that case the low pass filter should be set fairly low, to allow the sub to simply add where the main speaker falls off. This may make integration a bit easier, but you do not benefit from the potential of a high pass filter to lighten the work load for your speakers and power amplifier. So if you can, use a line level connection and a high pass filter. If your amplifier or subwoofer do not have one (few stereo amps or subs do), you will have to add one, either electronically, or in the form of e.g. the FMods from Harrison Labs. 80 Hz is a common frequency for this, and this allows you to use a rather higher low pass filter on the sub, and make it do more of the heavy lifting.