Yeah, for those who are considering the Meridian Explorer, I would discourage any thinking that it can't perform well in the absence of an external 5V power supply. Seriously, if you buy the Meridian Explorer and don't hear any noise or hum, then your USB port is most likely providing sufficiently clean power from your laptop or PC. The End.
But if you buy the Explorer (or any other USB-powered DAC) and you do hear noise, or hum, or you later just want to find out for yourself if things can be improved when using the portable DAC as a desktop solution, then go for it - spend another $199 for the iFi iUSBPower. (Say goodbye to portable operation, and say hello to the AC outlet.)
This reminds me of how I felt when everyone got excited about using a 15V external battery pack to power the Meier Audio Stepdance - unanimously finding that the Stepdance sounded better when running on 15VDC than when using an internal 9V battery - even Jan Meier said so. Next thing you know, everyone is thinking the Stepdance is no good without the 15V battery. Wrong! It's just better with the 15V battery thanks to everything that improves with an increase in power.
In the case of the Meridian Explorer, there is no guarantee that it will actually sound better with the iFi iUSBPower on your equipment - with any USB-powered DAC, depending on how clean (dirty) your PC or laptop's USB power is. It could easily be clean enough to make the iFi iUSBPower a waste of money.
Don't fix what ain't broken.