Is their any evidence that the SMPS in a Mac Mini audibly degrades sound quality?

[Hybride]

Ground connections between equipment as one of the base problems, yes. They interacts with the digital section of any dac including timing (hf) related distortions. Breaking the ground by using Toslink is an easy way to eliminate a part of these problems. Or using a USB galvanic splitter can be effective. The capacitive coupling of the (mains)transformers must be very low. I believe capacitive coupling of mains transformers is a main problem these days in audio PSU. High frequency distortions of the mains power easily pass transformers/PSU. My DAC is therefore batterie powered. But even a batterie can have its signature, because of its own internal impedance over the audioband and batteries can also produce some noise. A lot of money can be spend in changing SMPS's to lineair PSU's. It al changes the sound perception in a certain way. I think using decent mains filter can be more effective than changing a good designed SMPS to a Lineair PS.