Since you're monitoring two channels simultaneously, you could try subtracting them, as an alternative to carving out bandstops.

1. Drive both channels with the same time series
2. Tweak the filter module gains if needed to equalize the analog outputs
3. Apply SR785 user math to subtract the two channels (or A-B mode of a single input, if you're not using that already?)
4. Measure the residual, which should be the incoherent part containing DAC + coil driver noise

Subtraction can conceal certain annoying effects (like numerical noise or level crossing glitches) that remain coherent for two identical outputs. It might be worth experimenting with a differential offset or sinusoid, to try to break up that kind of coherence if it exists.