Something

pcm.upmix_20to51 {
    type plug
    slave.pcm "lowpass_21to21"
    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 {
    type ladspa
    slave.pcm "upmix_21to51"
    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 0 ]
            }
        }
    }
}
pcm.upmix_21to51 {
    type plug
    slave.pcm "surround51"
    slave.channels 6
    ttable {
        0.0     1       # front left
        1.1     1       # front right
        0.2     1       # rear left
        1.3     1       # rear right
        0.4     0.5     # center
        1.4     0.5     # center
        2.5     1       # subwoofer
    }
}
pcm.!default {
   type plug
   slave.pcm "upmix_20to51"
   slave.channels 6
   route_policy duplicate
}