Stx will make a slight difference, not much. Enough to not notice unless you do side by side test. I have owned both, the difference is very slight unless you need a headphone amp or modding ability. If you have unlimited money then go for it else your dx is about as good for most practical purposes. Such small differences can also be had with expensive cables though that might cost more, but just want to make you realize that the difference isn't noticeable except for side by side comparisons.
Is stx worth an additional 4k? Depends on your budget. For me it is. For most it isn't if you don't need the extra features.
Is stx twice as good as dx? NO
Rest assured, the stx is as good or better than most external DACs in the sub $150+ range but the dx isn't far behind. It just doesn't have the amp and other features.