well, the issue is pretty simple .. there is one answer to that, get an amp, if you really want something really cheap and effective, the apple dongle could surely help giving more power to them so they will be louder :)

i prefer optical since it sounds smoother than usb and it was pretty much always like that with any dac i have tried but i do like usb sometimes when i want some more energy. Really depends on the day :) its nice to have both options.