Valab DAC Question

Yes - that is the case on my and probably all Macs.

When you select the USB driver in the Audio MIDI Setup tool, you turn off the native "Built-In Audio" output which is what feeds the optical output.

FWIW I believe that the Valab prioritizes the USB input when it is live. The switch selects between optical and coax when there is no USB input. I say this because many DACs are set up this way.

To prove it would require a second source to test.

It makes it hard to A/B on the fly, but other then that it doesn't represent a problem for me.

I wanted to use the Valab as a source selector and was disapointed when I started to see limitations, but this is a huge disapointment if usb prohibits switching.

Sorry - I don't think that there is a way around it - I was hoping to A/B two different USB DACs - that's when I figured it out

Every multi-input DAC I have ever had gives preference to one input - in some cases you need to throw a switch, in other cases you literally need to turn off the source.

It seems that what you are describing is a pre-amp and a DAC integrated into a single case - which is a great idea

Perhaps someone else knows of such a unit

But don't give up on the Valab it's a terrific unit