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.
Nice build! Yeah. I have a Kustom KGA 10FX I won on ebay for 1 British penny! 😄 It's built in a way so that the amplifier can be taken out as stand alone and has a speaker cab output. I've been toying with the idea of doing something similar to this.