I guess the 250-Ohm version is the "safe" recommendation.
Usually the (big) price difference, between the 250-Ohm to 600-Ohm headphones is not worth it to some people.
I like to think buy the 250-Ohm and a cheaper sound card and spend the extra cash towards a better amp.
If money is no option, then get the 600-Ohm, if your willing to pay a for a little bit better.
As your looking at the DT990, it might be a little more worth it, as most people will notice the difference for the DT990.
Maybe get the Essence STX, but hold out on buying the DT990 600-Ohm until you finding a used one for a good price.
I already have an STX and I can get the DT 990 600 ohm and 250 ohm for the same price so that isn't an issue. I was just wondering which headphone would be better for the soundcard disregarding price.
If you can get the 600 Ω version at a decent price, it is worth giving a try with the Xonar STX. The card can output nearly 7 Volts (RMS, at the highest gain setting) into that impedance with very little distortion, and that should be enough for the majority of people. For comparison, many use 250 Ω DTxx0 and even AKG K70x/Q701 headphones from the FiiO E11, and think that combination is fine, while the STX powers the DT990-600Ω better.
Of course, choosing 250 Ω instead has the obvious advantage that it is reasonably usable from many other devices with lower voltage output than that of the Xonar STX.
I don't really plan to use it on many devices and am content just using it with my computer for now.
So I guess I should go for the 600 ohm then? Thanks for the input guys.