Thanks for that article! Looks like an interesting read.
In part due to output impedance which he listed an article about it above ( I didn't think this would be a thing in main stream receivers, but I guess it can be).
Also your motherboard does have a DAC, but doesn't have an amplifier to deliver sufficient power for higher impedance headphones. This is noticeable more so in the bass frequencies, since bass takes more power to produce (and thus suffers first before the mid and high sound frequencies). It is kinda like plugging in high impedance headphones into your phone or MP3 player. Ironically some headphones such as the Steinheiser 598 seem to be designed with this in mind, and sound great on unamplified channels, but too bassy on an amp.
The analogue signal can also be tainted with noise from the electronic components in your system. Having an external DAC gets those components away from the sources of noise.
1 Like