Perfect,

You should be able to see the noise relatively easy with the oscilosope on the pre-outputs (I'm not sure if they have to be loaded though). It will not be calibrated so you won't be able to tell how much noise is there in dB, but you'll see it. When the volume is at -80, you'll see almost straight line on the scope. When you turn to -79, you should see the noise to appear. Set the scope to around 50-500mV range. It would be more interesting of course to connect a spectrum analyzer to see what the noise actually looks like. I mean, is it totally random or has some shape.

If the noise does not depend on the volume setting, I wonder where is it coming from. From someplace after the preamps? After the volume control? Strange.