What frequency do I crossover subs

As low as possible as Danny points out but I'd recommend setting volume by ear as the off the shelf SPL meters don't work well at low frequencies. A correctly integrated sub is the one that calls attention to it self, in most systems with a sub its always set too loud, IMHO

Best of luck

Peter

Making sense at 60 hz: Maybe....your speakers are rated at 45 but how many db down....3,6, 10 db? Some manufacturers stretch the lower reading.

Scientifically: Your ears are the best instruments. Imagine someone looking at a row of TV's and asking someone else which one looks the best. Even then, if you ask 5 people you might get 5 different answers.

Advice (but using "your" ears): Interpolate. Set the x/o around 60 and listen to the lower midrange. Remember it's not a brick wall at 60. The sub will be putting out 120 and 180 hz also, how much depends on the slope. Then try it at 30. Keep doing that at lesser amounts above and below 45 til you get a sound you like.

My guess is that you'll find the best spot somewhere around 30 to 40 but there are many variables. Might be a good idea to invest in a SPL meter (Radio Shack around $50) to help with the volume and phase settings but start with your ears first.

If, you are not comfortable with your ears or you just want to be sure, the simplest way as onemug mentioned is to get an spl meter, you'll need to run seperate frequencies at 40, 50, 60, even 70 and see where each frequency peaks in your room. If you indeed have dips in the 60 to 70hz range, it may be beneficial to cross at 60... 60 is a spot though that peaks in many rooms, my guy instict would be to cross more like 40... good luck, Tim

Make sense to cross-over at 80Hz. 60Hz will be still to low.

This will depend a lot on the slopes of your speakers and subs. And yes, I agree that an SPL meter would help here. They are not expensive. Can't speak to the quality of them, but there are even SPL meter apps for iPhone and Android.