This brings up a question that's worth looking into: if you raise the center volume in 5.1, does that center info stay raised up when you switch to stereo? It could go either way. If it does, then switching to stereo when playing sound in the kitchen would give you enough center channel info for voice intelligibility.

If you're going to use preamp outputs to an outboard amp, it would not be a difficult thing, aside from the fact that you'd have to do some experimentation, to use a few resistors to mix the center preamp output into the left and right channels. Let's say you put a 10K resistor in series with the left, and another in series with the right, from the preamp output to the amp's inputs. Then run the center preamp output through a 5K volume control (pot). The output of the pot goes two ways: through a 2K resistor to the amp's left input and through another 2K resistor to the amp's right input.

I'm guessing at the values, but the point is that the pot, when fully up, would deliver more audio from the center than you get from either left or right, allowing you to raise the center info above the left and right info. (I can't guarantee that adjusting the pot won't change the levels of the left and right, but you'd be able to come to a balance.)