He is right, sort of. The S.E.X. is single ended so you cannot create a true differencial signal path from your DAC to your headphones. What you can do however is rewire the output transformers in such a way as to create a balanced output. With the impedance switch boards the intention is that you wire the speaker binding posts to the +/- instead of to ground, or in my case i wired the XLR socket in that way.
Note you cannot use both balanced and unbalanced at the same time, to switch between them you have to flick the switches on the impedance boards, or if you hard wired the entire setup your stuck with whatever you wired it for.