Control volume with DAC or Preamp?

You have a really nice preamp that adds no noise or distortion to the music path.  I would go with using the Benchmark pre for volume control.  You should then turn the dac volume to 100% so you get the full signal going to the pre.  The way it was explained to me is you want to push a full signal to the last car in the train before attenuation.

When I inserted the Benchmark pre into  my system, I  noticed immediately better sound at lower volumes and much much better and granular volume control than the dac.  You may have to adjust the pads at the XLR outlets to properly match your components but that is better explained in the user manual of the Benchmark.

The most important thing here is that typically and I personally don't know about the Directstream Dac but most dacs do it, you set the Dac at full volume, you will most likely bypass any DSP stage, DSD to Pcm internal conversions, and possibly even filters leaving the source mostly untouched, if your preamp is good I think that would be the best way to go

I have a Berkley Alpha in my small system (with preamp) and Berkley says set their dac to 51 (volume) when using a preamp.  There should be an optimal setting on the dac and you should control the volume with the preamp.  Volume control on the dac is tough to get right.  Listen to what happens to the midrange as you increase & decrease volume digitally.  

OP here. Thanks for the input. I experimented and found that, with the Benchmark preamp at maximum volume, I could hear a fluttering, vibrating noise from my speakers when standing close to them with nothing playing...sounded like the speaker cones were making it.  If I turn the Benchmark preamp volume down a little and the DAC volume up, the fluttering stops and the sound is clearer. 

Another option, try driving the amp directly from the dac.  How does that sound compare with the pre in the chain?