I’m sorry I missed it, particularly for Compute Module discussion, but had no choice. I am hopeful that either a Zynthian Mini Variant or Zynia (or both) can be based on a Compute Module in the not-too-distant future.
re - headphone amp:
I think this is single rail - VCC is DC IN + and VEE is DC IN GND, see the referenced, Creative Commons, schematic:
And it’s available as a partial kit or just a bare PC Board or DIY.