What do you consider when setting crossover freqs?

You are dealing with 2 curves and the idea is to blend them so that the result is a flat line. Not possible but that is what you are aiming for. For example, if the main speaker is -3db at 60Hz you would set the subs crossover at a point that would boost 60hz output back to flat. Lacking sophisticated measuring equipment (which is getting cheaper all the time) the best thing to do is set it by ear, as all rooms will boost or suck out the bass to some degree. Get a CD or LP with test tones at different frequencies and set the crossovers to produce the same level of sound at the different frequencies. Some subs have complex circuitry to do this for you but you can do it for yourself, it takes longer.

Girst I loook at specs for ballpark then adjust to my taste, the room is gonna be a big factor too. Its really gonna come down to material aswell.

One problem is that the bass measurement changes when you change seats. If you are closer to a wall or corner, you will measure, hear, and feel more bass. You get it all figured out, but then move to the other end of the couch, and you might feel like you need to make adjustments. I think that is why people get more than one sub- it helps even things out a little bit. But then you have to balance your mains and 2 subs. AAAAAAAH! Just make sure you sit in the same place.

For Bass, set it by ear. In my unusual system where the custom subwoofers are good to 350 or so and the mains are MG1.6, good to 40 Hz, the best setting depends on the music. I have a convenient way to change the frequency (a knob).

usually 10hz below the main bottom response so the blending does not leave a bass hump - start low and move up til you hear it and then back down a notch, is a very basic STARTING point

when all else fails try the two pink things then engage grey matter follow with bubbly