The iD44 can happily drive up to 600ohm headphones without problem. During the development of iD44 we tested a wide variety of headphones with a wide variety of impedances to be sure that the iD44 would sound great, whatever headphones you like to use.
If you prefer numbers, here are the maximum drive specs of iD44 at various impedances:
MAX LEVEL INTO 30hms:
MAX LEVEL INTO 6ohms:
MAX LEVEL INTO 600ohms:
If you'd like more information on how impedance affects power transfer from the iD44 to the headphones then click here.