I did measure the line-out before, it is as flat as the headphone-out (given the same signal feeds into the amp section). Again, I don't hear any mid bass hump even on my desktop speaker. The line-out doesn't have a particularly good soundstage and separation, causing the overall imaging to be focusing more on the centre. Could that be why you think it has a mid bass hump?
I seriously doubt that. Like I said, what I'm hearing is not subtle. Do you know if there's a capacitor or something in the X3's line out signal path that could be faulty in some of our X3's and may be causing this result?