Something

# 6 channel dmix:
pcm.dmix6 {
     type dmix
        ipc_key 1024
        ipc_key_add_uid false
        ipc_perm 0660
        slave {
                pcm "hw:0,1"
                rate 48000
                channels 6
                period_time 0
                period_size 1024
                buffer_time 0
                buffer_size 5120
        }
     }

# upmixing: 
pcm.ch51dup {
        type route
        slave.pcm dmix6
        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.0.5 0.5
        ttable.1.5 0.5
   }

pcm.duplex {
     type asym
     playback.pcm "ch51dup" # upmix first
}

# change default device:
pcm.!default {
     type plug
     slave.pcm "duplex"
}

# for aoss
pcm.dsp "duplex"

pcm.dsp1 "duplex"