Digging up a thread from last year....as the OP's testing mirrors my own experience concerning Dac Volume Control vs Pre/Integrated Amplifier.
Like the OP, I own a Hegel HD25. My amp happens to be a Belles Aria.
With the DAC set to 100%, my music gets "very loud" at 9-10 o'clock. It would be blowing the roof off at 11 o'clock. My curiosity was prompted by a thread on the Harbeth User Group site, going back to 2014. In it, Alan Shaw was adamant (as only he can be) that amps should be working towards the top part of their volume pot movement for best sound quality. i.e loud should be experienced between 2-3 o'clock.
Shaw went on the say that there must be something "seriously wrong" if you're spending most of the time listening in the 8-9 o'clock range.
Provoked by this, I spent some time over the weekend testing different combinations. Here's what I found:
1) To achieve 2-3 o'clock on the volume pot for very loud music, I'd need to reduce the DAC volume to around 50%. This produced a muddy, muffled sound quality. Obviously no good.
2) Around 75-80% on the HD25 means "loud" is achieved somewhere in the 9:30-11:00 o'clock range. The music is clearly more rounded and analog sounding than full 100%. It's more enjoyable and listenable for longer periods. It still apparently has the detail and punch of full signal, without the glare. I like it.
3) This evening I'll experiment a little more with 85-90%. Could be the sweet spot, as the OP and others have found
4) 100%, I have concluded, is too hot. I've never experimented before as I accepted Hegel's recommendation that 100% is the preferred setting when using a preamp. I never really cared about the volume pot position. Having tried lower DAC settings and higher amp settings, I'm convinced this is the best solution.