Cross over frequency ???

I think you laid it out very well Sean. Just one thing added, if you use something like a 6 db slope the transition will be very smooth but you may find out that you may negate part of the effect of the subs because the satellites are still active for too long and the sub sounds muddy because it extends too high. As Sean said, then you definitely want the crossover point to be different between the subs and the satellites.

No, not two separate crossovers just different frequencies for each section. If you could you would leave the subs at 80 hz but crossover the monitors at lets say 100hz. Twenty -four db is pretty steep. That should be a fast enough slope. If your monitors don't like 80 hz try raising the crossver gradually upward. The higher the bass the more likely it is that your sound will get muddy.