It would be interesting to find out if a DAC upgrade would improve your present set up, but I suspect the limiting factor of your present system may be your speakers. Improving the quality of the signal entering the speakers won't help if the transducers receiving the signal are at the limits of their resolution.
For $600 you should be able to pick up some good desktop or standmounts. You can get powered desk mount speakers and consider selling off the electronics, if you want to pursue that route.
For $600 you should be able to pick up some good desktop or standmounts. You can get powered desk mount speakers and consider selling off the electronics, if you want to pursue that route.