This line out issue gets raised yet again. Let me throw out some hints as to why it may be a red herring.
People were paying close to a $1,000 for this limited edition version of the DD iDevice DAC with a military spec opamp. But it was a DAC: line out only, no volume control, intended to feed an external portable amp. So what was it doing with an opamp?
Look for a high end DAC without a gain stage after the DAC chip. Even one...
Go test one of those tiny Sabre-based USB DACs that outputs right of the DAC chip, e.g. the Stoner Acoustics UD100 (which I own). Ask yourself: why does this not sound so great?
No doubt, with most DAPs it is desirable to have a direct tap off the DAC chip for the line out because of the limitations in the internal amp section (i.e. distortion and/or coloring the sound) even though a proper home DAC always has a gain stage after the DAC chip. However when Calyx say that setting the volume to 100% on the M creates a true 1.25 Vrms line out, they may know what they are talking about. A big reason that the M is so exciting is that they understand Sabre implementation - that is both before and after the chip.
The distortion numbers as published are certainly insanely low which bodes well for this having a truly fine 1.25 Vrms line out. Time will tell.