I've often seen it said that some people find the DX100 disappointing because music they thought sounded great on their iDevice sounded poor on the DX but that seemed more because you get such clarity and detail poor recordings offer all the faults disguised on other DAPs. I don't think that is elitist just fact. And I have to say that I have been gradually re-ripping my older CDs that were done at 128kbps on early software with favourites now on flac.
But none of that to me explains muddy! I recall you saying that you found 1.3.7 muddy. Did you revert to 1.2.7 to check? Personally, I'm happy with 1.3.7 as I've already noted, but short of A/Bing with someone else, I think you should contact iBasso. They may recommend you use their tool to reinstall your FW first. Certainly worth a try. That will reformat everything to replicate a new DAP that's never had any previous version on it.