The source impedance (headphone output impedance) of the SSL 2 series is 10 ohms.
SSL 2 series is designed with professional audio quality in mind, which means that there is ample power to drive any headphones from the SSL 2 series headphone output.
For optimum performance, a typical ratio of headphone impedance to source impedance should be 8:1 or better.
This leads to an optimum impedance of 80Ω or greater; this covers most professional headphones.
Concerning headphones with much higher impedance:
Typically headphones with very high impedance (250 Ω, for example) may suffer from a lack of power when used with a very low source impedance output.