Thanks for your interesting arguments! They have led me to do the comparison myself again (I'm not sure if I've ever done it with the Mojo or just with the Hugo).
I used my best headphone amp, the Meier Audio Corda Symphony, and the FiiO X5 II (via S/PDIF), with my (
modified) HD 800, EQ preset «HD 800».
I started with the amp in the chain – and was surprised how much I liked the resulting sound. I coudn't find any glaring faults in it. The Mojo solo sounded rather dull and lackluster in direct comparison. So back and forth i went. The longer I switched, the more I found the Mojo solo to sound clearly better – in the sense of more honest and accurate. There wasn't one single objectifiable criterion in wich the amplifier route was better to my ears. The bass was tighter, better «controlled» (accurate) and faster, without being less dynamic or punchy in the least. Through the amp it was definitely slightly slower, although not in an irritating way or to an irritating extent – it even evoked the vision of luxury, like the whole further spectrum: Everything sounded like polished, rough edges were slightly smoothed – certainly the amp in the chain isn't a bad thing for bad recordings. However, to my ears the original HiFi ideal is better represented in the Mojo alone – a wire without gain replacing the added, technically redundant headphone amp. What also struck me is the different soundstage characteristics: wide and expansive with the amp, more modest and with much better depth and imaging the direct connection.
Conclusion: I can absolutely understand the preference for the illogical «double-amping» route, resulting in a more elegant and forgiving sound characteristic, but as I see and hear it, it provides a fake, euphonized image of the original recording. Rob Watts' saying really has something to it: «Some people like harmonic distortion» (the one that any amp adds to the original signal), it's not even hard to do so, but fortunately I'm sort of immune to this seduction (which saves me quite some money). The Mojo is even a special case, in that it is the driest sounding Chord DAC/amp I know. That's why I also had my reservations against its sound and preferred the Hugo to it most of the time. On other occasions I was fascinated by this very trait, a sort of hyperrealism and hyperprecision. With an amp in the chain you don't have to deal with this sometimes magical and sometimes irritating feature, as it is simply cleaned away.