Add a digital coax in /out tube pre-amp

Why?  What’s the point?  Why would you even do that?  Just get a DAC and feed it into the analog inputs in your preamp.  I don’t get this at all. 

As a pass through? Or do you mean to install a dac inside the pre? 

who, what, where, when, WHY?

What are you trying to do?

How would you select that input?

Tape Loop. Perhaps a small external box, run thru the Tape Loop?