Reading audio transformer datasheet
The DC imbalance will apply : is your capacitor's leakage current &lt; 3 mA? Yes ... done.

I would assume, power delivered to load, it's the normal way of rating a transformer. But I would also assume that's the sort of power required to start saturating the core and produce (unspecified) level of measurable distortion, rather than the level above which the transformer is destroyed. Presumably the load is 600 ohms or higher; 1mW into 600R is 0.775V so you are well below 50mW in normal operation.


Driving it from 200R instead of 600R should be fine; it will mainly improve the LF response (which is none too impressive according to the datasheet : telephone rather than pro audio grade).