If you don't understand that little tutorial, I'll summarize what you need to do.
Switch to the 5.5 volt output and try to adjust your amp to 5.5 volts input as well.
Setting it to the minimum (8V) will only allow your amp to produce about 50% of it's rated output while setting the amp at 5.5v will allow it is full unclipped output.
You can use the method Scooby described by setting the volume to 3/4 and then moving the gain until the onset of distortion then back it off slightly ('til the distortion stops).
