The Audient id14 software allows you to decide what you send to the headphones and the physical outputs. The software is pretty flexible. Not as much as my trusty old Saffire Pro 40's MixControl (which now serves as additional preamp into the ID14 via ADAT), but it's good.
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Thanks very much for the suggestion, I've just been on with support @ Audient and they recommended the id14 / 24. Said it's possible with the 14, but might be better off with the 24 as it has more flexible routing options for the 'phones. I've had a few suggestions for the Saffire so will give them a shout too and see how their setup works.
I didn't mention my ID14 is mark 2 Just in case the hardware is relevant to the software.
focusrite 2i4 shares its headphone bus with output 3/4 if i remember correctly, which might be what you’re after if you don’t need more than 2 line outs?
Going to have to edit the post because I found a solution in the end... went for an Audient iD24