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.
-- Ronin1973
I was a bit suspicious about this but RME states this about the Fireface UCX II:
Voltage Ranges
Line Out 1-6, unbalanced (mono jack), unloaded:
+19 dBu: +9.5 V, +13 dBu: +4.75 V, +4 dBu: +1.7 V
I think it's pretty solid. I'm informed that modular works at 10 V ranges. Please let me know if I'm wrong about these. Here's the manual (page 43): https://www.rme-audio.de/downloads/fface_ucx2_e.pdf