not to spark up a debate but more a learning.. and remembering that each system is specific to the drivers/amps/wiring/etc so this is not a "one size" statement:
I seem to remember for years from my car audio days that high pass was best started at 110hz as the rolloff would extend down and subs were best at between 80-90 and again, due to rolloff, would be a smooth transition. again, just starting points and every system is different but good guidelines.

thoughts?