[atledreier]

For level and timbre matching the front and center speaker you could use a simple sound effect, preferably one with some bass and full frequency.

Play it like this:
Left -Right - Both - Center - both - Left front - left surround - left front/surround - right front/surround - right surround - right - all

Or some other sequence that make sense. A simple effect can be placed anywhere in the sound field easily in Premiere when outputting to 5.1. just go pair by pair for level and timbre. Should be easy to implement, and very effective.