The source impedance (headphone output impedance) on SiX is approximately 1ohm.
For optimum performance, a typical ratio of headphone impedance to source impedance should be 8:1 or better. This means that SiX' headphone output requires a pair of headphones with 8ohm or greater impedance; this covers most headphones.
Concerning headphones with much higher impedance:
Typically a pair of headphones with a very high impedance (250ohms, for example) would suffer from a lack of power when used with a very low source impedance output. SiX is designed with professional audio quality in mind, which means that there is ample power to drive any headphones from SiX' headphone output.