I sent an email to iBasso to ask them to make it selectable from a menu selection. Let's see what they'll say. Apparently, according to Sorensiim, it is available on the DX100.
I suggest if you can to make the request, here is what you can ask:
"Can you please allow us through menu option the ability to pick the DAC roll-off audio band behavior, that is the wm8740 slow roll-off or fast. I believe Soren mentioned you made it available on the DX100."
The more making the request, the most likely they'll implement it.