In short: the sub-bass is somewhat rolled-off, mid-bass bumped up, upper-midrange forward and a bit aggressive, midrange as a whole is congested, highest frequencies are rolled off.
. . . I thought I was going crazy since no one else is reporting this kind of sound output.
No, miceblue, it's not just you. I'm having similar experiences with the X5 paired to my Shure SRH840 full-size headphones (44 ohms, 102 dB/mW). I know I've been spoiled with the HO of my Marantz SA8001, but my goodness--how disappointing!
Edit: miceblue, which firmware are you using? My unit arrived with version 1.1 installed; I may revert to 1.0. At present, the X5 is definitely muddy, congested, and sibilant.