can voltage variation affect the performance of preamps and cd players or dacs, especially if power supplies are not "robust"?
There are undoubtedly many subtle ways in which line voltage variations can have subtle effects on the sound, especially if the power supply and other aspects of the design are not robust.
Just to mention one, higher line voltage would figure to raise internal operating temperatures slightly, at least for anything in or close to the area of the power supply, and temperature is a parameter that is fundamental to semiconductor performance.
That said, I would think any such effects would be very subtle (within a reasonable range of line voltage, say 110 to 125V), and would not be consistent or predictable across different designs.
Regards,
-- Al