This actually makes sense! Before you added the new VT-1c, engaging 6.1 Surround would cause the 1050 to redirect some data from the left and right surrounds to the empty rear center, creating the sense of weakened surrounds that you noted. You were correct that the surround data is distributed across all three surround speakers (left, right, rear center), but doing this would reduce the levels of the left and right in order to generate data for the rear center. Having no speaker connected there would result is a loss of data, and thereby a sense of decreased volume level from the surrounds. Connecting a speaker to the rear center output would solve the problem.