No volume control for some channels in 5.1 on Terratec 5.1 PCI (Fun) with CMI8738-MC6 Chipset

I've got a problem with my soundcard not being possible to control the volume for some channels.

I am trying to set up my soundcard to play the sound via the analog jacks either 5.1 or 2.0 depending on the device I choose for output.

I used the speaker-test utility to check out my config (which is based on the alsa soundcard wiki entry for cmipci and the mythtv wiki).

speaker-test -t wav -c 6 -D surround51
-> The sound is on the expected speakers, but the volume control is not ok. Master controls front left and right, PCM controls front left and right and rear left and right, there is no way to control center and LFE with alsamixer.

speaker-test -t wav -c 2 -D 20to51
-> Works nearly as expected, except that one rear channel doesn't provide any sound. And there is no volume control for center and LFE either.

Tests only involving two channels like speaker-test -t wav -c 2 -D test (or -c 6) work like expected. Sound on Front left and right, control with master and PCM is possible.

The output of the alsa-info script is here: http://pastebin.ca/525222

I would be *very* happy if you could give me some tips how to hunt this thing down.

Greets,
Mario Minati