Am I right in saying that most digital devices are designed to have low output impedance?
No. They are likely deigned to have good sound and or good performance (which are not always the same...). Some have tube outputs and are designed to have as low an output impedance as the tube will permit... at any rate, low output impedance isn't usually the goal although its usually desired.
One of the main reasons we've sold line stages over the last 25 years and right to this day is that analog volume controls often outperform digital volume controls. With an active preamp you then set up the DAC to have the volume all the way up so there is no resolution lost, and then control the volume with an analog control.
The problem with a passive is the volume control is a series element with the output impedance of the source. This means the source impedance is raised quite a lot insofar as the interconnect cable between the passive control and power amp is concerned (and also as far as the power amp is concerned). So controlling the interconnect cable, even if the control is 10KOhms is a lost cause. That is why passives are so susceptible to interconnect cable interaction.