I can't read the specs for your front speakers, but from what you've written it appears that they are about ten db down at 45hz. That usually means they start losing output at a higher frequency than that.
If they were acoustic suspension (no port) the rolloff is usually fairly gradual so starting at 80 to 100 hz as suggested above is probably best. Since they are are bass refliex (ie a vented box with a port) the frequency curve is probably a slight bump in the low end followed by a fast rolloff below that frequency, so a lower frequency for the low pass might give better results - maybe 50 or 60. If you can find a frequency response curve for them you might be able to see what frequency they start losing output.
Of course, the bass response will depend on your room and where you place the subwoofer. I'd say try setting it at 80hz and if there's an unnatural bump at that frequency adjust it down. Note that many people like that unnatural bump.