I've seen some vague claims that an XLR microphone-to-usb that's powered solely by USB will necessarily sound worse than one powered by a separate power source (a wall wart) because the preamp will necessarily be of lower quality?
Is there any objective reasoning behind that?
It won't necessarily be lower quality, but it could be.
USB ports can quite happily deliver 5v at 2 amps - but if your PC/laptop/mac is under heavy load you may find that it drops a bit, or the 5v isn't very pure so you may get fluctuations in the power supply to your mic adaptor.
The same thing happens with a mains supply, but it can be easier to filter out the 50/60Hz mains hum than to filter out the wide range of high pitched interference you can get from components within your computer. You can also get a higher voltage easily from rectifying and smoothing mains current, as opposed to having to upscale from your USB's 5V with an inverter.
High-quality linear components generally need more than 5V supply, insofar the claim is reasonable.
However, it is nowadays not a big deal to include a DC-DC converter to attain a higher voltage. Of course this needs more current, but USB offers rather plenty of Amps. Then you also have enough headroom to smoothen out most interference with fixed-voltage regulators etc.. Some people will claim that this never achieves as clean results as a traditional 60Hz AC trafo, but there is no real, scientifically sound reason why this should be so.
Most USB interfaces apparently go the DC-DC conv. route, otherwise they also couldn't provide 48V phantom power.
In theory, there are electro-physical reasons as mentioned by Dr Mayhem.
In practice, you will find AC power supplies almost only on larger multi-channel USB interfaces and hardly ever on a one-channel or two-channel device. If you do some research and explore reviews about audio quality of USB interfaces, you will find out that the quality of semi-professional USB-powered audio devices is notably good nowadays. Renowned manufacturers use the same microphone pre-amps as in large multi-channel devices and thus deliver comparable audio quality.
So in theory, there is a difference, but in practice, the only difference is that small one-channel or two-channel devices are USB-powered for mobility reasons without a notable loss of quality.
