Really? Almost everyone has said the X5 sounds warm. Even the reviews state that claim. What makes you come to that conclusion?
Strange. The X5's treble is extremely refined to my ears, moreso than the HM901's. Check your music files.
This is where I disagree, because the X5's highs come off as laid back, but not in quantity. The highs sound effortless to my ears, having great extension with no "detail spikes". I'm right now A/B'ing it with 4 other TOTL DAPs, and it's probably the smoothest sounding of the bunch. Not dark or warm, smooth.
This is not about flac or something else, it is about a good mastering and bad mastering. Sadly, the most metal/rock albums have bad mastering. I am huge fan of that kind of music and I found the masterings are pretty bad. Even remastered editions are not really day and night difference, though a clear improvement compared to the old ones.