The Tape Out on the 970 is for analog input only. There's a note in the manual that says you can record analog-analog OR digital-digital but NOT digital-analog or analog-digital.

The only crazy idea I can come up with would be to use the headphone output as a pseudo-line output. The manual doesn’t say but I’m assuming that the digital inputs are routed to the headphones also. The manual is clear that the headphone volume is totally independent of the main and remote volume controls.