Regarding the FiiO amps vs the Xonar_DG sound card, I would say try your new headphones (whichever ones you get) using the Xonar card first, since you already have it. That sound card should be fine for the headphones you have mentioned. If you need a portable amp, that is another thing entirely, and one of the FiiO amps, like the E6 or E11 should be on your list of amps to consider.
If you want another suggestion for headphones to consider, I like the Creative Aurvana Live! quite a lot. Take a look at this review over at Innerfidelity: http://www.innerfidelity.com/content/creative-aurvana-live-classic-reincarnate