I have a 5.1 soundcard, and use ALSA as sound system. I'm not really satisfied with the ALSA mixer, cause the only two volumes that I can control are front/rear-left and front/rear-right. center and subwoofer are always 100%. I'd like to be able to control every single channel.. is there any way to do that? maybe an ALSA plugin or something?

TIA

since you have 5.1 sound working then you have the controls i bet.
alsa controls are specific to each driver so often a generic mixer is not showing the whole deal.
use the command
amixer
by itself to see the availabe controlls
what kind of soundcard do you have.
my cheapo sblive shows
a major control called Surround
then
wave surround
center
LFE
wave center
wave LFE
i have a GUI mixer that shows this stuff in a little box labled surround
qamix
you can use qamix and its configuration files to show all mixer controlls or a few in any design.
don't ever mess with the settings it uncovers that just go on and on and are numbers !