pcm.upmix_20to21 {
    type route # as automatic routing does not work, use type route
    slave.pcm lowpass_21to21_plug # 'route' outputs integer data, but float data is needed for ladspa.
                                  # the additional plug will output float data
    slave.channels 3
    ttable {
        0.0     1       # left channel
        1.1     1       # right channel
        0.2     0.5     # mix left and right ...
        1.2     0.5     # ... channel for subwoofer
    }
}

pcm.lowpass_21to21_plug {
    type plug
    slave.pcm "lowpass_21to21" # now, relay further to ladspa
}


pcm.lowpass_21to21 {
    type ladspa
    slave.pcm lowpass_float # ladspa will also output float data, but hw or route devices will need integer data
    path "/usr/lib/ladspa"
    channels 3
    plugins {
        0 {
            id 1098 # Identity (Audio) (1098/identity_audio)
            policy duplicate
            input.bindings.0 "Input";
            output.bindings.0 "Output";
        }
        1 {
            id 1672 # 4 Pole Low-Pass Filter with Resonance (FCRCIA) (1672/lp4pole_fcrcia_oa)
            policy none
            input.bindings.2 "Input";
            output.bindings.2 "Output";
            input {
                controls < 300 2 >
            }
        }
    }
}

pcm.lowpass_float {
    type lfloat # converts from float to integer data
    slave {
        pcm "surround51" # your slave device here
        format "S16_LE"    # the format ladspa uses
    }
}

pcm.!default {
    type route
    slave.pcm upmix_20to21
    slave.channels 2
    ttable.0.0 1
    ttable.1.1 1
}