The Hegel has a pre out, so yes it can be used as a preamplifier. I also confirmed with Hegel.
I looked at a photo of the H390 tonight and saw the Line Level Outputs for use as preamp and DAC. Too bad there isn't an Analog Out in addition to the Digital Out, to allow the H390 to be used as a source component.
You did your due diligence and now moving on to new adventures. Good luck and hope you enjoy the process along the way.
One final thought. You mentioned buying a tube power amp to use with the H390.
Another option is a tube integrated with HT bypass or other method of using it as a power amp, to provide the flexibility to remove the H390 in the future, if desired.

