8. Why can a mechanical sound be heard from the X7 when earphones are plugged / unplugged?

A: The X7's headphone output reaches the headphone socket through a relay.  The relay would briefly break the connection between the amplifier and the headphone socket when the X7 is powering on and off, to avoid power on/off transients from reaching the headphones.  The relay is an electro-mechanical part that makes an audible "click" when working (corresponding to when the relay is engaging and disengaging).