[Lucien Nunes]

You only need a few milliwatts to drive a pair of phones! And it's not just about level - the source impedance affects the frequency response.

AFAIR IEC 61938 recommends a source impedance of 120Ω to drive any phones regardless of their impedance and historically many phones were designed to give a correct frequency response with this source. But more recently I believe a low impedance source has become the accepted norm for typical consumer 32Ω phones and some people prefer the response that way, with higher damping. There is no correct answer these days; impedance, sensitivity and Q all vary widely amongst popular designs.

To deliver say 5mW into 32Ω phones with a 120Ω source resistor (and that is enough even for lower-senstivity phones) would take just 1.9V swing at the amplifier output. 1W into 8Ω from your low impedance secondary is 2.8V so not a million miles off. You might want to load the amplifier more not just to protect the output transformer but to move the load impedance nearer to the optimum point for distortion.

Conversely, the 600Ω winding would be delivering up to 39 volts at 2.5W which would require an L pad not just a series resistor. I would stick with the Lo-Z output particularly if you want to go for the lower-than-120Ω-source sound.