What frequency do I crossover subs


My speakers go down to 45hz and I've been told to crossover the subs at 60hz. That doesn't make any sense to me. I would think the crossover point would be between 40 and 50hz. Is there anyway to figure this out scientifically?
taters
I replaced all the knobs on my current fave guitar amp with Chicken Heads so I can see the settings instantly. I say Chicken Heads for everybody! It just makes sense.
Martykl,

I do agree with you that the scientific method or DRC does help getting to the goal faster but the final jugdment is still by ears.