It has been now pointed out to me what I should have spotted a mile off - in the circuit diagram the inputs are shown feeding straight into blocks marked "Dual ADC", and everything beyond that is happening in the digital realm, 'gain' included. So when recording in 24 bit, it's possible to digitally raise the levels above what 24 bit can store, leading to clipping. In 32 bit float, the digital gain will not raise the level above the 1500dB (or whatever the figure is) that 32 bit float can capture. All in all, it makes the 16 and 24 bit options a bit pointless, unless immediate output in that format is required. Unless I'm misunderstanding.