I'd suggest 80Hz, but you can experiment with a lower crossover setting to see if you prefer it slightly lower, assuming all your speakers can accommodate.
From my own experience with my 20-39 PCi and Boston Acoustics VR3 mains that go down to an fairly accurate 35Hz, I'd say the SVS is likely to be better at handling bass between 40-80Hz than most speakers, but as I mentioned, you can always experiment, see what you think and go from there. As long as the speakers can handle it, it boils down to a personal preference at that point... -TD