I was playing today a little bit with the DPLL setting an I could swear it sounds better lower the setting is. More transparency with the lower setting. From the ES9038 datasheet: "For best performance, the DPLL bandwidth should be set to the minimum setting that will keep the DPLL reliable in lock". The DPLL controls the bandwidth. Setting the bandwidth to its lowest value results in maximum jitter reduction. If the bandwidth is too low, the DPLL will loose lock and you will hear dropouts. If it is too high, it will lock safely but with some sound degradation. Thus the usable lowest bandwidth setting is one where no dropouts occur. I have been doing testing with the MacBook Pro and there were no dropouts even with the DPLL switched off. I guess the default "high" DPLL setting has been chosen in order to be compatible with widest range of sources, such as smartphones, tablets and laptops. Still for the better sound quality, I would experiment with the lowest possible setting, as long you hear no dropouts.