Add a digital coax in /out tube pre-amp

SPDIF to and from an old preamp?

Silly.

Get a DAC with enough inputs for your needs (remote controlled if you know what’s good for you) and plug its analog output into an aux input on the preamp

Or get modern an buy an all-in-one like an A20, only  $22K.

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?

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

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.