Note that I have only used AC on my TIU's in passive mode. I use DC on a fixed channel on the bench for loading sound and chain files. I don't know how they're react to passive connections with DC, though I suspect it should be the same as AC.
In passive mode, the setting of the variable channel to fixed mode should have no effect, so I don' think that's an issue.
FWIW, I think I'd have a 22uh choke between the power supply and the track, the TIU output should NOT go through the choke. I'm guessing the DC supply will have a pretty significant capacitance, and that likely will affect the DCS signal since you're impressing it on the output of the DCS signal generator. Here's three that should be suitable: High Power 22uh Chokes