Something

# Expand stereo to all channels including LFE
pcm.20to51 {
  type route
  slave.pcm surround51
  slave.channels 6
  ttable.0.0 1
  ttable.1.1 1
  ttable.0.2 1
  ttable.1.3 1
  ttable.0.4 0.5
  ttable.1.4 0.5
  ttable.1.5 0.5
  ttable.0.5 0.5
}

#
# Control device (mixer, etc.) for the card
#
ctl.20to51 {
  type hw
  card 0
}

pcm.!default {
    type             softvol
    slave.pcm       "20to51"
    control {
        name "SoftMaster"
        card 0
    }
}