The guy in this thread basically had the same questions as I. I thought about bumping it, but it might suit me best to have my own thread.

It is pretty hard to set the crossovers without some sort of analyzer; however, you can use some real defined test tones and a DMM as long as you remember that at the crossover point you will be down 3dB. Basically adjust your gains, play a tone well into your passband with the crossover off. Then play a tone at your crossover frequency and start to rotate the crossover visually until you are close while using the DMM looking for the 3dB down point. Of course as a check since you know your slope you should play a tone an octave higher to make sure your attenuation is pretty close.

If I want my High Pass crossover point to be at 3.5khz @24db slope, then playing a tone at 3.5KHZ should be half the wattage of what frequency? Should I expect full wattage at 4khz?

It's a bit tricky to figure out exactly where the crossover will start sloping the response down in order to get to -3db at the crossover. A safe bet would be to just go another octave up, i.e. 7khz.