I'm not sure Loopback is the answer here. Peace is just a GUI for EqualizerAPO, which is installed to specific windows output devices. There's no input and ouput that you can select like in most audio software.
To get around this I think you would need to use two different outputs in windows, with EqualizerAPO only installed on one of them. This does mean you would need to manually switch output devices in windows.
To do this, decide which two windows output devices/software playback channels you want to use, and then use the EqualizerAPO configurator to install EqualizerAPO to one of them. Then, route the software playback channel with EqualizerAPO installed to your IEM output in TotalMix and the other output without EqualizerAPO to your speaker output in TotalMix.
Then use windows to select the output device/software playback channel without EqualizerAPO installed and you should hear the non-EQed audio on your speakers, then select the output device/software playback channel with EqualizerAPO and you should now hear your EQed audio in your IEMs.
If you want, you could install EqualizerAPO to both output devices/software playback channels and use device specific EQ profiles. That would allow you to EQ your speakers as well.