i found that when using the external USB DAC with hires output, the player will force using 32-bit 192khz whatever the music is. And the setting in Poweramp can't make any change.
(i have change the setting to 24bit 96khz but it doesn't work)
But i found a APP named USB AUDIO PLAYER PRO, it can output the right sample rate and bit depth.
I hope Poweramp can add a function like this then we can freed from the damn Android force SRC. It will be a great improve!