Which version of the V72 is in question here? The original NWDR-version used an input transformer CTV72/1 with 2 x 1:20 ratio (2 x 400 turns pri. and 4 x 2000 turns sec.) with a 400kΩ secondary termination resistor. The nearly identical first generation Siemens V72 uses a very similar input configuration, but without any terminating resistor for the input transformer secondary. Finally, there is the Maihak V72, which followed the 'Braunbuch' specifications with the CTV72/1 input transformer termination of 250kΩ. The V72 was followed by several versions of the V72a, all of them featuring slightly different input transformers and terminations. The series last 'official' implementation was the V72b by Siemens, which used a balanced push-pull circuit topology from input transformer (1 : 13.5) to output transformer (23.2 :1) and using two dual triode tubes instead of the two pentode tubes used in the previous versions.
In order to provide an unbalanced line input to the V72 or V72a I propose to disconnect the input transformer, making sure the grid of the first tube is returned to ground through the terminating resistor, and then providing the audio input through a suitable decoupling capacitor (22 nF to 100 nF). If necessary a variable resistor or fixed resistive pad could easily added to accommodate most signal sources.