If you're using a Mac you can aggregate audio interfaces. Simply put, you can connect multiple audio interfaces to your Mac.
On PC, Windows only likes to have ONE audio interface. There are workarounds here and there. But this is it.
The ES-8 acts as an audio interface. So if you're using your actual audio interface to control your monitors or service other gear, you're now out of luck unless you can connect everything up to your ES-8.
With a DC coupled audio interface with MULTIPLE OUTPUTS, you can use spare outputs to drive your modular synth gear and use the others for normal audio. The issue you'll run into with the audio interface is the range of voltages that it can reproduce.
Another option is to use a MIDI to CV interface like an FH2 that acts as a MIDI interface. Or a MIDI to CV converter which simply takes 5pin or TRS MIDI and converts that to CV.