I recently got Shure SE846 IEMs which I'm connecting to a 2015 13' rMBP. The IEMs came with an inline volume attenuator, basically this.   At first I plugged them in without the attenuator and there was some hissing + high sensitivity to volume changes. I later learned both of these issues are...

Thanks for the help! This is making a lot more sense now.

Ok, and just so I understand, a headphone amp would be useful because it provides enough voltage to counter the increase in source impedance, preserving the source? Would it make sense to get an amp that is plugged into the wall instead of a portable amp?

Here's my setup: I have a JEC TC-716 stereo source selector (audio switcher). This switcher has 4 pair of RCA jacks and 2 stereo 3.5mm jacks. I use all four RCA jacks, one from my computer (which has a Realtek ALC889 audio chip), one from another computer in the room with the same specs, and one...