When it comes to setting up a crossover in a sound system, 80 Hz is generally considered to be the best frequency to use. There are a few reasons for this. First, 80 Hz is low enough that it will allow most subwoofers to reproduce bass frequencies without any problems. Second, 80 Hz is high enough that it won't cause any major issues with the sound quality of the mids and highs. And finally, using a crossover at 80 Hz will help to minimize any potential phase issues that can occur when different speakers are trying to reproduce the same frequencies.