This is not so easy topic how it could look. I already discussed with Jussi digital vs analog volume control topic in this thread one-two weeks ago. See my first post on the topic
https://www.head-fi.org/threads/hqp...-settings-rolling-thread.968141/post-17978221
and then the discussion starting here:
https://www.head-fi.org/threads/hqp...-settings-rolling-thread.968141/post-17986325
One aspect is subjectively perceived sound quality and other one is to avoid damaging of your headphones (or loudspeakers) with accidental full volume level.
Regarding sound quality, my suggestion is try yourself which combination of digital and analog volume level sounds best for you. It depends on your DAC and headamp/preamp noise floor level, sound signature of both and your listening preferences. You can adjust volume range in HQPlayer to allow finer settings in the area of your preference.
Suitable volume range set in HQPlayer of course depends on your headamp gain, sensitivity of headphones, if you are switching between different headphones and if you are the only user. I am using only HE-500 and I have set -12 dB as the lowest volume level and 0dB as the highest. I have usually set something around -6dB and doing fine adjustments to differences between recordings on my headamp. My headamp volume knob level is usually somewhere at 3/4. Even full headamp volume would not destroy my HE-500, but would be inconvenient with loud recordings. But I can imagine that when switching between headphones of very different sensitivity, there is a risk of omission which one were the last plugged in and too much power could be dangerous for the more sensitive ones.
Theoretically ideal situation would bring a headamp without any volume control, whose gain would just fit your headphones sensitivity so that 0dB HQPlayer level would set the maximum level you wish to listen to. Then you could use only digital volume control. Advantage would be that nothing is lost on headamp analog pot. As soon as you have analog pot in action then it is about which attenuation (digital or analog) means less loss. It is related to noise floor level of your DAC vs. headamp and the level of attenuation needed. That was the main point of the linked discussion. So the theoretically ideal headamp/preamp gain level is that one where you don't need to attenuate much.