I never really doubted the quality of the DAC on the STX. The problem is you can use the STX only on a desktop computer. Currently that's all I need, but if I end up getting an external DAC in the future, wouldn't it make the STX kind of useless? However, if the STX really is the best option in the < 300 EUR price range, I'm ok with getting one. I'm just not fully convinced yet. I've never used DS and I doubt that I would need it.
Yes, it's stationary and an external dac and amp would basically render it useless except for the Dolby Surround part, but then we're talking about better equipment and totally other sums of money and therefore it's irrelevant. I would say it's hard to beat for its money. I also find it to be more bang for the buck then its little brother since the STX's better amp is nice to have if you happen to buy higher impendance cans.