I'd set it to 0dB and see what the auto setup does. If the amp sets it around 0, at least you have scope to turn it up or down if needed. Very occasionally, an amp might set a sub to -7db or -9dB or something stupid. If it does this, set the sub to +6dB, then run the auto setup again. At least then the sub should be about -2 or -3dB and leaves you with the ability to turn it down by the amp if needed.