I've been looking for an inexpensive upgrade from my Soundblaster Z, would this pair better with DT770 250ohm and HD558's? I assume the lower output impedance would be a lot better for 558's at least.

Anyone have any advice or reccommendations?

I currently have a Sound Blaster Z that I'm using with my DT770 pro 250 ohm. So, would the Dragonfly sound better, and does it have enough power for 250 ohms? 

If you don't mind boosted bass and some peaky treble, then the DT770 pro sound like a good match. They are closed ear, and do a VERY good job of isolating noise. You won't even hear someone speaking into your ear when you are listening at a decent volume. But unless you absolutely need the noise...

I think it basically comes down to whether you want open or closed ear, and which sound signature you would prefer. Other than that they are basically identical.

Now that I think about it, my ears do touch the drivers. But I've had mine for 6 months, and only really noticed now. There is soft material on the insides that probably won't bother you much on both the 770s and 990s.

I have DT770s myself, and I can vouch that they are VERY comfortable. My only minor complaint would be after a several hour gaming or movie session, they can get a little hot, but nothing too uncomfortable. I'd assume the DT990s have a similar level of comfort, considering they are both the same...