Would these be a worthwhile upgrade from Sennheiser PC360? I can get them new for under $300. Also will my Xonar ST drive them properly at 600 ohms?
I haven't heard that model my self, but the Xonar ST should be able to drive them. You might have to ask on head-fi tho. But I sure the ST would be able to handle them.
The ST can drive them fine. I don't have any experience with the mid-fi Sennheisers so I can't say how big of an upgrade they would be over the PC360. As Claymore advised, you could ask on Head-Fi.
The DT 990 will be a clearly noticeable improvement over the Sennheiser you have. I had the Sennheiser HD 555 once, those are 1 or 2 notches better than those you have, and the DT 990 pro I have are a considerable improvement over those. Yet the also available 250 ohm version might be a better choice as those are easier to drive and thus less dependent on other equipment. And the Pro version sounds the same yet is around 1/3 cheaper. The 250 ohm Pro version you could get for under 200$.
