Think of it from this point of view, if you get the Titanium HD, it will power your headphones easily enough that you won't need an amp. Like I said before, your headphones are quite easy to drive, and the little bit of amping they could use to improve, the Titanium HD takes cares of that due to having some grunt.
Like Mad Max said, different soundcards have different sound signatures. An easy example of that is when comparing the Essence STX with the Titanium HD, the first has a brighter and slightly harsh sound, while the latter has a smoother sound. All this with default opamps, that is, because on those cards you can swap opamps to change sound signature to something more to your liking.
