Tom, interesting way to beat the ground loop that you came up with. Yes, when you connect both the left and right line level inputs twice the voltage is fed into the sub amp and it sums them, resulting in a 6dB increase in level(a doubling of voltage is 6dB, compared to 3dB for a doubling of power). Note that this doesn't mean that the sub's maximum loudness capability is increased by 6dB; it remains the same, but a lower setting on the volume control is used. A similar result follows from feeding just one input, but increasing the volume control to let twice as much voltage in.
