hi River161, Jack and all.
bah...after write long post was needed to login again and i lose all... 
well the resume of the lost post is:
~80% or more of the basses are in the centered image of stereo source.
without Center channel we lose only the 'center image' level comparing with surrounds as all is present in the stereo source(L&R) but we lose the level of the subs to LFE as all channels are redirecteds.
without center you lose less in authoreds cds/dvds via coaxial/optical but using spam multichannels from pc to amplifier you lose more as don't have basses redirections in this last case.
no matter if the voice, guitar, trumpets or other is in only one side only, the basses are centereds in ~80% of the musics and will be redirecteds...think when the bass is in one side only and is imaged in center at the same time or when have 'mad' transitions between channels like Hendrix... when the bass is passing from one side to other, what will happen with the LFE level in the middle of the transition? the same question to voices and instrumments in transitions.
i don't know(don't remember) any music that have 'center image' empty where all instrumments and voices are in extremes only.
if exist any music only with extremes, then we can let the center channel empty and only in this case!
then...we have to use the center IMHO(it's not a 'law', just personal argumments and taste)
this is why i posted the AC3 and the seperateds waves.
to do your own seperation, create an empty stereo wave in any wave editor, copy and past the L to left, R to Right and save...you have my source to do the extraction.
cheers!
