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# I only have 4.0, but something like this might work
pcm.ch51dup
{
        type route
        slave.pcm surround51
        slave.channels 6
        ttable.0.0 1   # left      --> front left
        ttable.1.1 1   # right     --> front right
        ttable.0.2 1   # left      --> rear left
        ttable.1.3 1   # right     --> rear right
        ttable.0.4 0.5 # 50% left  --> center
        ttable.1.4 0.5 # 50% right --> center
        ttable.0.5 0.5 # 50% left  --> sub
        ttable.1.5 0.5 # 50% right --> sub
}

# My 4.0 routing looks like this:
pcm.ch40dup
{
        type route
        slave.pcm surround40
        slave.channels 4
        ttable.0.0 1
        ttable.1.1 1
        ttable.0.2 1
        ttable.1.3 1
}

# To make that the default alsa device
pcm.!default
{
        type plug
        slave.pcm "ch51dup"
}