If I were doing it, I would set them both to 70 db to start out. Then I would play some sample music or DVD and see how it sounds. If it is too much, I would back them off. Too little, juice them up.
Also, it is not enough to get them to the right DB level. You also need to get an equalizer to flatten the frequency response of the subs.
For me, I have mine playing the same DB as my speakers for 2 channel and I crank them up to be 3DB hot for a DVD (non concert).