[IDEA] Hide volume knob

[the_bertrum]

I think the reason it is important to show that the volume is fixed at 0dB is more for those who usually use the volume knob, so they are alerted to the fact that they've made a configuration that will blow their ears off if they don't have a downstream control.  I suppose it could be argued that the volume knob disappearing says that as well, I think it better to say it explicitly in the UI for safety.

As for a polarity inversion switch, I wouldn't call it essential but I can't better chance on a polarity blind test.