Powered speakers require a line level signal which is lower than a headphone requires. By turning the volume down sufficiently you can avoid overloading the input of the powered speakers so it will work. However the circuit required for headphones ( a low output power amp) is very different that a line level output so it would sacrifice potential sound quality. Some DAC/Headphone amps also have line level output. Whether it is a different circuit or just tapped off the headphone amp only the maker would know.