Problem with multichannel LADSPA plugins

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Hello,

I am trying to setup a lowpass filter for my subwoofer channel. I used the configuration template which is given in the ALSA wiki (http://alsa.opensrc.org/SurroundSound) as a template.

My asound.conf works when I use it like it is given below (sound from all speakers). But when I uncomment the second LADSPA plugin for the lowpass (plugin number 1), then I will only hear sound from the subwoofer (so only channel number 2 is processed lowpass_21to21). The other channels seem to be muted. I also tried the same setup which is given in the Wiki mentioned above, but this does not work at all (the delay_0.01s plugin is called delay_0,01s on my machine; it think it's because of the German locale; that's why I am using the ids for the LADSPA plugins now).
I tried to set the policy of my first LADSPA plugin (number 0) to 'none', but this did not help. I was still hearing only the sound from the subwoofer.
I tried to use 3 plugins for the 3 channels, each with policy none, but this did not work too.
The ALSA documentation I found (e.g. http://www.alsa-project.org/alsa-doc/alsa-lib/pcm_plugins.html) is not very helpful at all (I am not sure how to set these bindings inside the plugins correctly).
When I connect upmix_20to51 directly to upmix_21to51, the sound is working fine (of course without lowpass).

I would greatly appreciate any hint about what I am doing wrong.

Regards,
Benjamin Eikel

Package versions (I am using Debian unstable):
libasound2	1.0.13-1
alsa-utils	1.0.13-1
alsa-base	1.0.13-2

My asound.conf:
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.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 ]
        #    }
        #}
    }
}

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