For rear speaker output in winamp for most of us you need to use the "speaker fill" mode , however this can cause rear speaker distortion in music , it sounds crap. However i found a fix. Download the plugin "A3D_Output_Plug-in" for winamp , go into winamp options-Output change the output from directsound to the A3D plugin.
Go into General Purpose , change to the A3D setting and config it , tweak the settings to your desire
, mine is like this below
Reload winamp , Perfect quality sound through all speakers no more distortion and works great in vista and windows 7!