If you told the avr there is no sub just leave/set your speakers to large/full range, just don't exceed their limits with the volume control would be the way I'd start. Crossover is active when you set speakers to small and indicate you're using a sub. Maybe tell the avr there is a sub and then you can use the crossover to protect the speakers but I'd try large/full range first, possible your avr will set a high pass filter for your speakers with no sub enabled but I don't know, never really tried that.