I do politely disagree with your subsequent justification of the term «adequate». There's not much subjectivity in it, it has a rather absolute characteristic in the sense of «worthy» or «of equal quality». In other words, your original statement was like: «The HD 800 doesn't sound good (enough) with the Hugo or the Flow.»
Of course I dispute that. In my experience with the Hugo and several amps I've tried with it, the HD 800 driven by the Hugo sounds clearly better than with any attached amp. This to my ears and particularly according to my personal sonic ideals. It's even technically justifiable, as a
statement from Rob Watts shows (linked several times already, but apparently overlooked by some).
On the other hand I do understand the wish to modify the sonic balance of the HD 800, even when modified. I absolutely sympathize with that. But I would never use an amp for that purpose: It's absurd to amplify a signal that's already strong enough and as clean as it gets. You'll barely beat the Hugo's headphone out in terms of harmonic distortion and output impedance. I for one use the tool that's effectively meant for that – and avoided by the most vocal amplifier advocates like the plague.