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