I have the datasheet for the ESS DAC, but I'm under NDA. But I can at least say that the THD correction is on by default. I'm about to order a Khadas Tone Board, so I can hook up my logic analyzer on the I2C bus. I should be able to fiddle the settings of register 22-23 (which is on by default) with a Bus Pirate to turn off the compensation and then measure before and after. But first I need to replenish my wallet because this forum has put a serious dent in it. Topping D50 DAC, Behringer UMC202HD and now the THX AAA amplifier. Ah, and a subscription to Roon of course!
I don't want to disassemble my D50 (yet,) because you know, music.