I may not have any IEMs that are as sensitive as the ZSR, but I haven't heard any hiss with any of my IEMs (12Ω - 80Ω), when using these budget DAP/DAC/Amps.
The output impedance is determined by the value of the resistors on the circuit board, that are in series between the output pins of the op-amps, and the output jacks.
NOTE: There will also be output series capacitors, so you'll have to trace the circuits from the jack, or the op-amp, to find the series resistors.
These devices have 100Ω series resistors.
If you want to reduce it to ~9Ω, you can solder 10Ω 1/4W resistors on top of the 100Ω resistors.
Reducing the op-amp circuit gain, just allows you to turn the Volume pot up more, so that it's operating in its more linear, and better channel tracking range.