Line out into the delay, then out of the delay into the fx return in the DSL. Got it.
How loud are you setting the output level on the attenuated Super Lead?
Have you tried swapping the instrument cables connecting the delay, and cleaning the jacks on the line out send of the attenuator, and...
Bummer... Definitely an Impedance/Line/Gain Staging Level issue. If the signal is too hot, you'd get terrible clipping distortion into your pedals. If that's the case-- dial back the Line out level on the attenuator and turn up the volume on the DSL on the clean channel
Does your loop on the...