I never tried the digital out. All I know is that you need a special cable wiring (see qls page).
As I understand the amp settings only effect the headphone out and balanced out. Not the line out.
On the two headphone out there is a very subtle difference in amp mode when you listen with higher impedance headphones (>150ohms).
I had a small blind test session with my son (10 years old and nearly new ears

) and he agreed that with double high there is more stable bass that with i.e. high voltage mode.
This effect is very subtle.
When I tried the wav1.0.6 the first time after listen to the wav1.0.4 and 1.0.5 for more than two weeks (daily about 3-4 hrs), there was a obvious advantage in clarity with wav1.0.6.
It was some kind of awakening for me because I never had a feeling about how I can recognize the impact of jitter. But wav1.0.6 was the answer: there is much less jitter.
Try to listen to wav1.0.5 for two weeks and then switch to wav1.0.6 and stay on the same playlist. You will get what I mean. And then remember yourself that all the superior and positive reviews out there based on fw wav1.0.4 /1.0.5.
It’s crazy.