You are mixing 2 different things. Generally you stop worrying about wall reflections and modes above about 300 hz. The crossover depends on your other speakers. You want your subwoofer to take over before your other speakers reduce their output too much. In general you want to set the crossover as high as possible on the subwoofer and let the AV Receiver crossover take care of things. In general 120 hz is a good place to set the crossover on the receiver 🙂
