I may be wrong here, but here's one possible explanation. With the sub set to off, the towers are also taking care of material from the center and surrounds (assuming they are set to small).

Here's a possible test for you: try a CD with the 950 set in "Stereo" mode (so that the center and surrounds are not active) and see what happens as you change the crossover settings. Then switch to "5 Stereo" and repeat -- the bass going to the mains will likely vary based on crossover points in this scenario, because full-range signals are going to surrounds and center and the crossovers and re-directing bass to the front speakers.

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