Originally Posted by headwhacker
1x for your iems 4x for your headphones. O2 using a wall adapter can put out 7vrms max (4.5Vrms on battery power). DX90's LO is 1.7 Vrms. 7 / 1.7 is approximately 4 so 4x is the max gain you can set O2 for DX90.
By default O2 has 2.5x and 6.5x gain settings. If you need the high gain, O2 will distort/clip the signal past 4x and you limit how much the volume pot can travel. Eliminate the unwanted gain so you can make full use of the volume pot at high gain without worrying about clipping.
For the sake of completeness, the EHP-O2 gain is set at 1x and 5x and to get a 4x mod, will need to wait for parts.
I was also told by the manufacturer that 1x will drive the DT880 600ohm adequately (unless I listen to it at a relatively loud volume).
So based on the 7/1.7 = 4, 4x being the max gain that the O2 can do for DX90:
I think the way around it would be to NOT use the line out port when I need to use the 5x gain, but use the headphone out port at LOW gain (1.3; 7/1.3 is approximately 5.)
The amp should arrive early next week, hopefully it all pairs well.