Once one has the data on hand of the pre amp and amp respective impedances and the freq response of the main speaker, how does one practically decide the high pass crossover point (80Hz in your example)? Is it typically the-3dB point of the main speaker's low end of the freq response? Should it be somewhat higher than the -3dB point thereby creating some overlap between the main speaker's bottom end of the freq response and where the intended sub woofer kicks in? Whats good practice to come up with the crossover point? Or does it not matter so long as its within the sub woofers crossover range?
Since I don't want male vocals coming out of the sub I chose 100 Hz and 24 dB/octave. If your slope is not that steep on the sub you have to choose a lower crossover point.
I would not go to the -3 dB of the main speaker, try to stay half an octave above that especially if there is a peak before the fall as there often is. There are lots of trade-offs to explore. Experiment if you can. If your satellites don't go low then you have to go higher with the Sub. If they go low and you have amp power go lower in the crossover. Listen to each speaker separately.
In my ESL system and with QUADS I recommend 100 Hz, 24 dB octave in both directions.