Control volume with DAC or Preamp?

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?

Adurerca- I did that and it sounded very good.  But I wanted to use a turntable and a tuner so I needed the additional inputs.  I started looking for a preamp that would be like connecting my DAC to the amp, give me the necessary inputs and accept balanced cables.  The Benchmark checks all those boxes.  Sounds about the same with the Benchmark as a direct connection to the amp. 

I have a PS Audio BHK preamp and DirectStream DAC.  I set the DAC at 100% and control volume with the preamp.  Works for me.  

I have a pure analog McIntosh system were I often control the volume using the MC2600 power amps gain controls and the 31V preamps volume control.  I can fine tune the listening level better that way.