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I don't think there is anything wrong with your connection. What you need to do is to mute your amplifier. You may try switching off the amp and see if you still get the music from the headphone. Some amp. design may allow signal to pass through even when the amp is off. I have the same situation as yours in my Tucana II. Signal can only pass from, say the Aux to the TAPE OUT when the amp. is switched on. I suspect there is a relay to direct which input signal is to be connected to the Tape out.
Alternatively you can insert a Y splitter to your DAC output and then connect one output to the power amp and one to the headphone amp. Hope the above can help.