I also have the H390 and used the USB input from my Innuos Zenith/PhoenixUSB source quite happily for a year or so.
However, I had the itch to upgrade, so took a risk on the Denafrips Pontus 12th DAC, plus Cardas Clear Reflection XLR and an old Ecosse power cable.
Having thoroughly allowed the DAC to settle I have been doing a little AB comparison, and can confirm that the Denafips is a step-up from the Hegel's internal DAC. Deeper bass, more resolution and detail without fatigue are the most obvious benefits.
Of course, the combined cost of the Pontus and Cardas is not insignificant, and I could quite happily have lived with the H390 by itself.
Another consideration here is the new big brother, the H600 which early reviews indicate it has a much improved DAC. Comparing the H390 and DAC with the H600 without external DAC would be interesting