The balanced output should be about 4V which is enough to drive something like a Sennheiser HD-600 or similar to reasonable levels, but not “loud”. The downside is that while the design includes a ‘High current class A hybrid output stage’, the output impedance is something like 100 ohms - as I understand it, they put a resistor in series with the output to protect the output buffer from a shorted output. The 300 ohm impedance of many Sennheiser headphones is a reasonable load for this, but on the limits. The only real way to tell how they will work together is to get a set of headphones and a custom cable and try it out.
J.P.