Headphone Direct Drive
Sorry by insisting in a some way off-topic route, but...
I have just tried and the levels are now far more satisfying on the headphones directly attached to the P2 pins on the RCA (via the 22uF coupling caps) and there is less distortion. Bass sounds good. This is still 8 bit audio remember.
75-ohm DAC pins can be ganged together to lower the impedance, you know.
Perhaps we could get rid of that headphone amp chip.
For two paralleled-DAC outputs, ~37.5 Ohm drive, 22 uF is only ~5% of the required capacitance.
Simple calculations shows ~420 uF minimum, for a -3DB cutoff rolloff at 10 Hz, so as not to "strangle" too much the bass response.
It seems like we should make a board which gangs four pins for each channel. Each pin goes through a separate 220uF series cap and each group of four (of the other sides of the caps) sums together to form left and right channels. This would result in about 19 ohms source impedance, which should be about perfect for driving headphones.
I need to get some 220uF caps and a 3.5mm stereo jack, in order to try this.
A lot of careful design went into the DACs to ensure they would be good for audio. Maybe they should just drive the headphones directly.