DC doesn't work for the variable channels. However, passive mode might work, though I don't know how the variable channels would react to DC on the outputs.
1. The fixed channels pass-thru the DC power from input to output - red-in to red-out, black-in to black-out. The variable channels use transistors switched on/off synchronized to the incoming 60 Hz AC cycle to chop the input voltage to create a lower output voltage. When you set it to fixed (no variable reduction), the TIU must still turn on its transistors on each cycle; there is no relay or whatever to simply pass-thru the input to output. OK. Now the question is what the TIU does if it does not see the input AC cycle and only sees DC. Since it is looking for the AC cycle, will it wait forever and never turn on its transistors? I don't know the answer but I suspect it would do just that and you'd get no output whatsoever.
This is correct been awhile since I looked over the dc side of things I have removed my statement.