mwc,
I'm fairly certain you are getting a big standing wave because of the room dimensions. It's very common. Try listening to the 63 Hz tone and moving around in the room with the sound level meter to measure the peak and null.
As to using a lower crossover at 80 Hz, apart from avoiding localization effects, there is a rationale that the smaller woofers in your main speakers will have better control for reproducing upper bass from 100 Hz up than the larger-diameter driver in a subwoofer. In other words, you may experience "tighter" bass by keeping the sub to frequencies of 80 Hz and below.
Regards,
