Yes, I doubt that leaving the gain at lo/med/hi has any adverse or benefecial effects to the DX50. I leave mine at low gain and don't adjust it unless I'm using higher impedance/low-sensitivity phones using the headphone out of the DX50 only. When connected to the C5 it's on low gain, just because I like it to be there
When connected to the C5 the headphone out will be the C5's. The headphone out of the DX50 won't be used unless you plug your amp or phone into it.