Lexicon MX400 input gain

[Reader2]

I use a MX400 in my synthesizer setup. It’s an extremely useful unit. However I can’t seem to kill the input signal using the input volume knobs. For instance my Oberheim Matrix-1000 is fed into side A, which is set to delay. I turn the volume knob 100% down and I still get delay coming out of it.

Is that just how this unit is? It would be great to control the signal from the knob on the front.