First, I don't have Hi Res files, so that has never been an issue for me, but I recently got the Fiio x5 III and have been reading forum posts about it non stop for 2 weeks. Any difference between the models and how Poweramp works could be due to audio driver implementation by Fiio. I'm not saying that's the issue, but I think these players have a lot of moving parts.
On my X5 III, I am using a custom kernel that makes quite a few internal changes. Then in Poweramp, I disabled DVC and changed the negative gain applied when DVC is turned off to 0. Now Poweramp sounds like Fiio to my ears.