I noticed the same on the first day I received my D2 Boa.
I noticed that a mini-mini partially inserted into the input jack produces sound from the DAC even when the amp is turned off (pot clicked all the way left & front blue light off). This appears similar to the line-out function on the python and other amps from ibasso, except that it doesn't work with a plug fully inserted into the jack. Can anyone else confirm this operation? (It works with headphones too -- I tested without them on my head to avoid hearing damage, although the output level was not that high actually.) Any idea how to get this to work reliably? Is there a different type of mini-plug that will capture this output? I would love to have the option of line-out with this amp (I would have bought the Python but I wanted the USB charging capability).