It very much depends on room acoustics, your fronts and subs and personal preference. Try 40, 60 and 80 and see what you prefer. Also try setting the sub's cross-over as well. I find setting my sealed subs to the same cross-over frequency as the AVR's cross-over works best for me in my room.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated