well I just studied the internal spec of the Tone2pro, and it seems that it consumes atleast 570mA nominal (just the USB decoder, not considering the internal audio/heaphone jack amp)
hence it shows the reason why your phone drained quickly…
I suggest something like 10,000 mAh to provide enough power for atleast 16+ hours of continuous listening, its purely your convenience, just choose something that can output atleast 5w of power steadily.