The VCP-1 has a volume control, doesn't it? (I'm going to assume for the moment that it does.) The signal for your mains (and only the mains) is being rounted through the VPC-1 and it's associated volume control. That would lower the volume of the 1050's output on the mains as compared to the volume of the surrounds and center (which all skip the VPC-1's volume control), giving you the significant change in level between mains and surrounds. That's my guess, at least.

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