Volume level chain

Generally I'd avoid attenuating the digital signal and keep the laptop and DAC at 100%. If you're trying to balance the level between preamp inputs so that the DAC channel isn't too much louder than another component then I'd use the volume control on the DAC to trim the level to match the other components but use the preamp to control the overall level when you're listening... if you know what I mean.
If your preamp has a particularly nasty potentiometer there's a chance that digital attenuation would be better but that is unlikely to be the case; when you attenuate digitally you are throwing away data (digital bits) whereas analogue attenuation reduces the signal voltage level. So the advice is to use the volume control which distorts the signal least and usually that is in the analogue stage.

1+ pragmas, digital volumes that are dithered are significantly better but still I leave all my digital equipment at 90% plus. Digital metering is a big plus in this regard. You can see where you are. In the analog world you just want to match volumes so you do not get a big surprise when you switch sources. Most modern preamps are hard to overload so I would try to keep them as high as possible matching volume to the lowest output source at it max volume. 
For people who do not have digital metering there is a device called the Audiophillio that is a USB to SPDIF converter that controls digital volume with loss of bits. Very cool device. 

Excuse me, without loss of bits.