Something

# 6 channel dmix:
pcm.!surround51 {
    type dmix
    ipc_key 1024
    ipc_key_add_uid false
    ipc_perm 0660
    slave {
        pcm "hw:2,1"
        rate 48000
        channels 6
        period_time 0
        period_size 1024
        buffer_time 0
        buffer_size 5120
    }
}

# upmixing: 
pcm.ch51dup {
    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.0.5 0.5
    ttable.1.5 0.5
}

pcm.duplex {
    type asym
    playback.pcm "ch51dup" # upmix first
    capture.pcm "hw:2"
}

# change default device:
pcm.!default {
    type softvol 
    slave.pcm "duplex"
    control {
        name "Software Master"
        card 2
    }
}

# for aoss
pcm.dsp "duplex"

pcm.dsp1 "duplex"