Unfortunately, when it comes to such a highly subjective hobby such as this, there is no definitive answer to your question other than only you can decide whether the price difference is worth it. Sony's XB 'phones are fun and uber comfy. Are the Ultrasones better? They certainly offer much more controlled bass, tighter presentation overall as well as soundstage that belies their size but they also are very aggressive sounding. Not everybody likes their strong presence in the highs. Personally, I find them too harsh sounding and actually prefer the 580 and 680.
There is quite the price difference between even the upper XB model and, say, the HFI-580. If they are worth that extra money is really your call. I will say this, if you plan on listening to nothing but lossy recordings at a lowish bit rate from a DAP then the Ultrasones might not be worth it. As you climb the Head-Fi ladder, source and typical listening environment play a large part in determining if it is worth opening the wallet wider for a pair of headphones.