Picture yourself on a tin roof with a premium grade hammer and a handful of very large roofing nails. You need to drive those nails into the roof as quickly and efficiently as possible, as you dont want to be up there all afternoon while your friends drink beer and ogle sexually attractive people who are also drinking alcohol.
For all its engineering brilliance, your hammer just isnt the right size and weight for the job (and the aforementioned attractive people know that), so you climb down and get a larger hammer from your *desktop* - the job is done in five minutes and you have a great weekend. Unless you are a certain-breed-of-Head-Fier, in which case you will spend as long on that roof as you need to, in the hope that you will be able to tell everyone that you drove very large nails with a very small hammer. Who cares about beer or bikinis when you clearly have so much on your plate ? Priorities, man.
I have the utmost respect for the engineering that has gone into the amp on the X, but I dont spend time trying to drive headphones that were designed to be plugged into a desktop amp - YMMV. As often pointed out in threads like this, its about more than volume. The X punches well above its weight, but (IMO) you dont need to change the sound signature and you shouldn't be trying to move mountains with it. For portable use, the added hassle of carrying an amp (any amp) simply to drive certain headphones just isnt worth it. Having a multi-tiered transportable rig is one thing, and makes sense for those of us trapped at a desk all day, but that isnt 'portable'.