I assume you mean distortion. That's what the DAC measures at and always has. Has nothing to do w/ Hifiman's implementation. That's the NOS signature people want to hear.
As you should know the LO not only bypasses the HO opamps but fixes the weird FR you see w/ the TF10 assuming your external amp is adequate. If you have a LO FR measurement attached to a properly designed amp lets see it. I haven't seen one anywhere to show me my ears are lying to me.
You do realize if I put 10ohms or more of output Z on the HO of any device and hook it up to the TF10 I can get weird FR curves no matter who designs it. Apple included.