Little Dot MK III volume control disabled when connected to V-DAC

I don't understand how this can be happening.  I wish it wasn't.

If I use this signal chain, everything works just like you think it would:

Computer soundcard line out -> Little Dot MK III input -> Beyer DT880

However, if I change it to this:

Computer USB -> Musical Fidelity V-DAC -> Little Dot MK III -> Beyer DT880

the volume control on the Little Dot does nothing.  Down all the way, up all the way, it doesn't matter, it sounds the same (really loud).  The only way I can control volume is in the source software.

I have tried various gain settings on the Little Dot.  I've tried connecting and disconnecting my PC speakers to the Little Dot's output.  Nothing has any effect.

Anyone have any idea what this is about?

Problem solved.... I had the input and output connections reversed.  It turns out the V-DAC was powering the headphones directly.  Oops.