The source impedance (headphone output impedance) on the SSL2/2+ is 10 ohms.
For optimum performance, a typical ratio of headphone impedance to source impedance should be 8:1 or better. This means that the SSL2/2+ headphone output requires a pair of headphones with 80 Ω or greater impedance; this covers most professional headphones.
Concerning headphones with much higher impedance:
Typically a pair of headphones with a very high impedance (250 Ω, for example) would suffer from a lack of power when used with a very low source impedance output. SSL 2/2+ is designed with professional audio quality in mind, which means that there is ample power to drive any headphones from SSL 2/2+ headphone output.