Audio Transformer Selection
I would use a 600:600 transformer just as is. 

Depending on the quality of the transformer, you may want to add a damping resistor to the output side. You can determine whether you need this resistor by driving the input stage with a square-wave signal and observe the output stage signal. Look for ringing on the signal.

You adjust the value of the damping resistor across the secondary of the transformer to minimize any ringing. 

In general, you want to drive the primary of the transformer with as low impedance as possible. This maximizes low-frequency response.