Never mind, this may be an issue with humidity or faulty microphone capacitors that has nothing to do with the AudioFuse. I'll report back if further testing reveals otherwise!
This is definitely an issue with the AudioFuse Studio's phantom power.
I have done a direct comparison test between the AudioFuse Studio (£799GBP) and an IK Multimedia Axe I/O (£299GBP), both with +48v phantom power enabled. Both were new, both tested in identical circumstances, in a silent, soundproofed room, with the input gain at half and then full level.
The Axe I/O was quieter at full gain than the AudioFuse Studio was at half gain.
I concluded that the AudioFuse Studio is unusable for vocals on even moderately quiet songs. It's simply too noisy when phantom power is enabled.
Is this just a bad batch of phantom power supplies or is it an issue with every AudioFuse Studio? I have sent this to Arturia tech support and await their reply. I will report back once it is received.
(I originally attached screen grabs of the test results, but they came out huge, so I've removed them)