From what I understand, you don't want to push the amp to high volumes as it intrudes distortions. Amps become non-linear as push it high. But then again, I've seen a graph of SS source and it showed under .01% distortion until clipping at certain voltage point(or volume point), which means there is not enough power to support. Headphones also reacts to high power push, it starts to distort more at louder volumes.
I believe the DX90 LO is directly from the DAC, and it's probably like the software volume control on your computer. I think the right balance of the two is most optimal.