It's a good point and I do have another Fiio to test this... Thanks!I can see there were more than one component changed, but any chance the signal source could be your issue? I know you mentioned it's a digital output but a defective source could be the issue you're chasing. Sorry if you've already tried, but another digital source to rule it out?
Actually, I did have an issue once that had similarities to my current one, where the sound would distort at a higher volume and I figure out it was a defective coax cable from the DAP to the DSP. I swapped the coax cable and the issue was gone. Not sure why it sounded fine while the volume was at moderate levels and not at higher levels, but that's what it was...
This damn hobby of ours... I love it, but now and then it really makes me mad! Lol