I am about to buy a Maverick Audio Tubemagic d1 plus. Should I plug it into the onboard optical (spdif) port on my motherboard or USB. Strictly talking about audio quality.
In theory they should be indistinguishable (it's just 1s and 0s that are traveling to your Tube Magic to be converted to an analog signal); it might just depend on the components of the motherboard (what materials it uses and how much material is used for the actual USB and Optical connectors). I haven't really been able to tell a difference on any of the DAC's I've used or owned when it comes to Optical vs. USB. The actual cable itself that you use to attach your computer to your DAC I think can make a difference (although that is a debatable topic with strong opinions on both sides), but the most important things obviously are your source material, DAC, amplification and headphone. Really what kramer5150 said is probably the best advice, I would listen to both and decide which one sounds best to me. I think USB is easier to use as it's more common (easier to find, upgrade, etc.).
I would guess(?) you will not notice a difference in the audio quality.
Optical can work with sound cards.
USB bypasses sound cards.