The THX recommendations are for sealed speakers. Although 80 Hz is commonly recommended, two things to consider:
1.) Set the xo 10-15 above the weakest speaker, example:
surround 58 Hz to 24kHz
mains 32 Hz to 25kHz
center 58 Hz to 25kHz
sub 18 Hz to 125 Hz
XO range 58 Hz to 125 Hz
2.) If a FR graph is done and there is a peak at 75 for example then setting the mains lower, 50-60 may get rid of the peak when running the system with a sub and speakers set to small
Note: defeat the sub XO by turning it all the way up or off. The problem with voices coming from the sub has not been my experience with MCACC.
derrickdj1's post in RP-280F System Crossover HELP was marked as the answer