This gets even more different when devices contain a digitally controlled potentiometer between the DAC and the amp. I believe Macs do this, with the main system volume control actually stepping down the voltage of the signal being fed to the amp. Since this is an analog control one doesn't lose bit depth when turning down the volume and thus this is a fine system to use.
Not all machines work this way, though. One way to check is to hook up an external amp and headphones, turn the computer's volume way down and the amp up to listen levels. If the quality is crap the it's probably just decreasing the bit depth. Or you can do a teardown on the sound pathway.
(Oh, if it isn't clear by this point, keep all your apps turned all the way up for best quality. Only turn them down on an individual, as-needed basis. All-software stuff has to decrease bit depth to decrease volume on a per-app basis.)
Not all machines work this way, though. One way to check is to hook up an external amp and headphones, turn the computer's volume way down and the amp up to listen levels. If the quality is crap the it's probably just decreasing the bit depth. Or you can do a teardown on the sound pathway.
(Oh, if it isn't clear by this point, keep all your apps turned all the way up for best quality. Only turn them down on an individual, as-needed basis. All-software stuff has to decrease bit depth to decrease volume on a per-app basis.)