Check the bass management settings for your front speakers. First, make sure that they are set to "small" - if they're "large" then the 1070 will provide no signal to the sub in stereo mode. (5 Stereo will provide a sub out in that case if your center and/or surrounds are set to "small.") Second, check out the crossover point for your fronts. If they are set to "small" but have a low crossover point, your music may not be producing notes low enough to dip below that crossover point. (Again, the situation would change in 5 Stereo if the center and/or surrounds are using a higher crossover point.)