While I do value your opinion more than most, my experience is simply not the same, and I am definitely not mistaking "thickness" or "quality" of the sound for bloated, wooly bass. What I'm hearing is not subtle - it is obvious and immeidately noticeable. More importantly, other X3 owners have reported exactly the same problem with the line out.
Rather than listen through a portable amp (something I haven't tried yet), I would suggest you try connecting the X3's line out to a home audio system.
I suppose it's also possible that some of us have received an X3 with a defect in the lineout? If it was just me experiencing this issue with the X3, that would be the obvious conclusion, but it's not just me.