AMIXER REAR CHANNEL VOLUME CONTROL

[]Trix[]

Hello!
I have a Sb Live 5.1 Digital with ALSA Drivers 0.9. I have two amplifiers. One for front speaker and one for rear speakers, but rear speakers don't work.
How can I enable or unmute rear speakers with amixer and set the volume level for them.
I tried using gamix and I managed to enable them with PCM Routing and by disabling 5.1 digital. But when I played mp3's it worked for one song only. When the next song in playlist started all the settings I made were lost. So I have been looking for the solution with amixer but found no manual or anything that would solve this problem.
Thank you in advance for your answers.