The sub is likely set to 120hz because one or more of your other speakers are set to 120hz so it goes by the highest one. If it were me, I would set the mains small and let the sub handle the bottom. The specs are quoted as +/-3dB at 33hz which says to me that 33hz is likely -6dB, so an 80Hz x-over is probably fine with them, but you could try lowering it and see how you like it. There will be no gap because as mentioned, one of your other speakers is likely set to 120Hz, which is why the sub is set high.