It really varies on speaker and manufacturers but most common will do good with the crossover at 80-120hz

These sometimes are used on subwoofers that dont have the single LFE Input. They sometimes only have a red and a white input connection. But sometimes they have all 3 red, white, and yellow LFE. If you use the sub out from the receiver which is 1 single rca cable then use the Y connector at the other end to connect at subwoofer in order to use both the white and red inputs it will give you little bit more sound. But this way the you have tune the subwoofer using the onboard equalizer on the selfpowered sub and not not the equalizer on the AV receiver. This is the old way of doing it nowadays almost all self powered subs have the LFE. Y can also be used to split sound into 2 for 2 different items reproducing sound like speakers or for two subs but it sometimes can weaken the signal because its getting broken down to 2 different sources.