The output of the both Moto X models was fine for me. It worked fine with all my earbuds and IEMs. It didn't work well with all my cans though, the more inefficient ones needed some help. The two models I currently own that they didn't work well with were the Superlux HD688B and the Audio-Technica ATH-M40. However, both of these are notoriously inefficient and even when listening at home (with all my head devices), I use an external amp - and both these models need higher gain settings on the external amp.
Some cans like the V-Moda XS needed to be at full volume depending on ambient noise level.
However, what i was doing was turning up the phone volume to completely drown out any ambient noise. When ambient noise is at a minimum, top volume isn't needed to due the clarity of these devices. Plus. both the Superlux and Audio Technica devices are not really practical for portable use. The Superlux look like alien sattelite dishes to the general population -either that or Breaking Bad lab protection. The Audiotechnicas are permanately wired with a superlong cable - which is great for the studio but awkward for portable use. Plus, both of these cans benefited from external amplification even when at home.
But for just walking around the park or store and listening to music, my Moto X 2014 is fine with almost all of my head devices. While walking around Wal-mart, sometimes I'm impressed with the clarity of sound. This especially happens when I'm listening to audiophile FLAC files ripped from MFSL or GRP digital masters. The two albums I have been listening to most are John Klemmer's Touch and David Benoit's Every Step of the Way. Poppy Jazz. The Moto X along with my Vmoda XS - and I can tell what brand of bass or guitars they are using and also the brand of guitar and which pickup they are using. Clarity like this in portable unstacked use was unheard to me until recently. Even James Taylor sounds awesome.
I can make a big improvement by using my external DAC and amp - but then I realize I'm getting further away from the definition of portable - same thing can be said if I want to wear cans that look like alien sattelite dishes.
The Moto X is one device and I no longer need to carry around a seperate media player, scheduler, tablet (or laptop!), or Navigation device. In some cases, the Moto X does these tasks better than the other devices, but in some cases, the seperate devices are better.
But the Moto X is good enough and the convenience more than makes up for any marginal decrease in quality.