Manual is mute on the topic, what does the little red "radiating speaker" icon on the front panel display mean?

It means that the pre-amp outputs are active.

When you want to use the headphone jack, you must hold down MUTE for several seconds. This will mute the pre-amp outputs and activate the headphone jack. When that happens, the red speaker icon will turn off. You hold down MUTE again for several seconds to switch back from headphones to pre-amp output, at which point the icon comes back on.

Thanks. Makes sense.