The key to getting some idea of where to set your crossovers is knowing what the frequency response of your speakers is. Specifically, what is the -3dB point? That is the point at which the speaker has rolled off enough that you are going to have an audible decrease in output. You want your crossover to be at or a bit above that point. In some cases, people recommend going up one octave band for the crossover (which would be doubling the -3dB frequency), but that's a rather conservative approach and stepping up at least 10Hz or so is usually a pretty reasonable place to start.

In the case of your speakers, we have the following:

• Gallo Ref. 3.1 -3dB point: 34Hz (22Hz with powered sub)
• Gallo AV C and AV M -3dB point: 55Hz
• Magnepan MMG -3dB point: 50Hz

I'd use a 40Hz crossover for the Ref 3.1 (or run it large with the powered sub in the mix), at least a 60Hz crossover for the AV C and AV M (with 80Hz as an option worth playing with), and 60Hz for the MMG.