The mid bass boost via line out on the x3? I don't follow the x3 thread but that doesn't sound right. I think it's the amp with the bass boost. The typical fiio house sound but I could be wrong. That's a pretty crappy dac implementation if it has a bass boost via line out.
The X3's mid-bass frequencies are noticeabley boosted. It's actually even worse through the line out, so it's definitely not coming from the headphone amp stage.