If it's anything like Cirrus' DACs (which AFAIK Apple contracted to design their own DACs), then you can defeat the impedance sensing auto gain with a regular old extension cable.
Plug the extension cable into the MacBook, then your headphones into the extension cable, and it should give you full beans regardless of headphone impedance.
Be aware though that that'll allow the Amp to clip when you push volume too far.
You can get short extension cables that won't get in the way:
View attachment 365950
Though sadly you'll have to repeat that ritual (Cable->MacBook, then Headphone->Cable) every time you plug it in.
Also, welcome to ASR!