Ok I know the problem and the soluton.
For bus powered DACs like the iDAC2, the Pi3 is a no go with high sample rates like DSD256 here, due to shared bus and ethernet.
Slim and Fernando don’t have this issue because their Pi3 isn’t powering the iDAC2 - their iUSB3.0 was providing nice clean power to the iDAC2 and the Pi3 doesn’t have to.
When fernando tried without the iUSB3.0 as above, he got the same pops I had.
So I just tested my old man’s same setup that’s been having all the issues above but this time with an Uptone ISO REGEN powering the iDAC2. And voila the Pi3 is fine with native DSD256 since it doesn’t have to power the Regen or iDAC2 and therefore the shared bus and ethernet is not an issue.
Sounds phenomenal.
Even a generic powered USB hub which disconnects power from the Pi3 will do and will power the iDAC2 from whatever external PSU you want. Don’t need a fancy Uptone or iFi hub to do the same thing, although they do other things of course also