What precision are the resistors? From what I understand the distortion of a R-2R DAC is limited by the matching of the resistors.

To reduce noise all the resistors of R-2R ladder must be 1% tolerance type. This is already mentioned in the BOM (https://github.com/dilshan/max2-audio-dac/wiki/Bill-of-materials).

Ah, I didn't notice the Git repository. I was interested in finding some information the relationship between the worst case THD for a R-2R ladder DAC versus component tolerance but I couldn't find an answer. I did look at the Seokris dam1021 which is also a 24-bit R-2R DAC available with 0.05%, 0.02%, or 0.01% precision resistors. The 0.05% resistor version is spec'd for 0.015% THD.

I'm curious about the large output impedance of 560 ohms. Headphones typically have an average impedance of 30-600 ohms but with frequency dependent variations in impedance throughout the audio frequency range. These frequency dependent impedance variations of the headphones impart an audible level frequency response if the amp's output impedance is too high. The rule of thumb is an output impedance less than 10% of the headphones' average impedance is necessary to avoid this frequency response.

Man, that's a LOT of resistors!

Yes, there are 100+ resistors in this board and soldering is the most time consuming thing!!!