The only way I was able to hear a difference was using test tracks that went below 90hz. The effect then became dramatically apparent. For my normal listening, the switch made no difference at all and at first I thought I had a defective unit.
+1. The bass boost is subtle, 3D much more apparent. However, the most impactful difference, even after adjusting for equivalent volume, is going to turbo mode. With that, my Senn. HD600 cans (300 ohm) bloom, sing and even grumble down low...if only that power mode weren't so power hungry ( I listen portably in addition to via PC -->DAC). The only potential negative of turbo. is that while clearly more detail and "in your head" sound presence I found turbo mode sessions to be a bit more fatiguing. For sustained non-critical listening, standard power represented a great balance between a great experience, battery life and minimal fatigue.
Overall, very happy with the iDSD.