What frequency do I crossover subs

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.

Most all of the above, but if you are running sub woofers to augument the bass of 2 way speakers, not to relieve the mid-range drivers of some of the load created by doing the bass, try running the main speakers full range and rolling off the subs as low as possible. A SPL meter can help finding the ideal cross over point as well as positioning them to take advantage of, or minimize room nodes/nulls caused by placement of the speakers, subs and mains, as well as the listening position.

Just a WAG, but if your speakers are -3db at 45hz, and your subs have a 6db per octave cross over slope, I would agree with Timlub and try setting the cross over at 40hz or lower. But consider the location of the sub will have a lot of influence on your decisions, its not just the cross over.