It's a matter of perspective, but I'd put my investment into something that gets more use out of it (i.e. the IEMs). And besides, unless you actually have qualms with the onboard sound, getting a new amp/DAC would be rather superfluous - rumours of liquid ecstasy and angelic choirs out of a 6x5 inch box in tandem to your already efficient headphones have been greatly exaggerated.
Yeah, I'm with TwinQY here. The general rule of thumb is that the closer it gets to your ears, the more of a profound change you'll experience... with one exception being the quality of your source files. Besides, I think that a DAC/amp would benefit more detailed headphones than they would - say - an M50.