I'm a little confused by all the complaints about the output level of the X3's line out. It's one of things I like best about it. Finally, a DAP with a true line out! It mates perfectly with home audio systems, and especially well in car systems.
If the consumer audio standard is 2.0 vrms, then it should be as close as possible to the standard, and I think the X3's 1.7 vrms is the closest of all portable DAPs on the market.
The problem some are experiencing with certain portable headphone amps is that those amps were designed to pair with the headphone out jacks (and the poorly implemented line out jacks) of iPods and similar mass-market devices. It's been mentioned in this thread that the iPod's line out is just .55 vrms. I, for one, do not need an audiophile DAP with the same kind of incompetently desigend line out as an iPod!
If your headphone amp doesn't mate with the X3, then the headphone amp is the problem. You can use your iPod with your headphone amp, which is what the amp was desinged for in the first place. Or you can invest in a properly designed headphone amp that will mate with a true line out.