This is a simple dac chip, contoller and op amp output. The only simple change would be to change the OPA2134 op amp, it in itself has a reasonalbe reputation and I have used it... If you want to do that, I would recommend an LME49720. It is a bit smoother and is found in a fair amount of high end gear and will switch out without issue. Outside of that, it could be that you have champagne taste on a beer budget.... I'm quite familiar with that myself.
Good Luck, Tim
Good Luck, Tim