Ralph, I think Bob raises a valid concern. This is basically a 1:1 transformer, not a transformer with a significant step-down ratio (which would obviously be inappropriate for a line-level interface).
So while adding a 600 ohm resistor to the secondary would make the transformer perform better, it would very conceivably cause problems relative to the output stage of the preamp (or source component, if that is what it is used with). If the output impedance of the preamp or source component is a significant fraction of 600 ohms, and is not perfectly constant as a function of frequency, then a frequency response irregularity would be introduced. And, depending on its specific design, the output stage might be unhappy providing the current that a 600 ohm load would draw, as well.
Or are we missing something?