How to play a recorded stereo mix back from Capture to the mixer?

Capture is able to record a mix on linked tracks 33+34 while playing tracks 1-32 to the mixer. This is perfect for making a roughmix of a concert or studio recording. But how do you play this stereo mix back to the mixer?

Unfortunately the UC Surface manual does not really describe the how-to and you don't see the signal coming into the mixer anywhere. You would expect tracks 33+34 to arrive at USB 33+34 (default routing for Aux In 1), but there is definitely no signal arriving at Aux In 1, Aux In 2, Tape In/DigRet nor Main L/R. The reason for this is that Main L/R sends its signal via USB 39+40 to tracks 33+34 and identically uses USB 39+40 returns to send the mix of tracks 33+34 back to the mixer. So the only thing you need to do is set input source of Aux In 1 to USB and select USB 39 (Left) and 40 (Right) as input stream.

The screenshot shows the settings. You can also use Aux In 2 or Tape In/DigRet for this task. Main L/R does not have any USB input source (returns), just USB sends.