How does the HD6 handle multiple inputs? Can I use Bluetooth and the optical input at the same time?

Both analog inputs are "open" or active, so you can leave 2 audio sources plugged in at the same time with no need to flip any switches! Both the optical input and the bluetooth module share the same DAC so using the optical input on the HD6 as well as streaming to the speakers via bluetooth would utilize the built-in DAC. This means that the HD6's bluetooth receiver takes priority, so if you start playing audio over Bluetooth the optical input will mute; optical will resume automatically a few seconds after Bluetooth is paused or disconnected.