I got a reply back from Pioneer, and they say the crossover is 120hz or 150? he says 150 at the end of his message, probably 120

"If I set the main speakers to Small, at what frequency threshold does
the signal get diverted to the sub?

Pioneer: The DVD player send the low frequeancies below 120 hz to the
subwoofer out put on the DV45A.

The other question is: The DV-45A allows me to set the Mains to large
AND have the sub set to 'ON'. Will the low frequency information (the .1
of 5.1 dolby) be sent to both the main speakers and the sub?

Pioneer: If in the movie there is low ferqueance special effects in
the fron channel only and no the .1, the front bass speakers will
response to the bass of the front special effects of the movie.
the .1 channel could do nothing during that time.

Each channel of dolby digital and DTS is full-frequeance

20 to 20khz in the large mode. If small from 150 hz to 20khz. "