Im not sure i could agree with this (the table).
While the HE-400 run great off my Schiit Asgard (max 1 W, but im quite sure it has some headroom as it can deliver 1 W even if you get some strong dynamic swing), my Fiio E17 dosent really run that well, even when USB plugged into the PC (note that the E17 I have runs a lot louder as DAC+AMP when USB plugged rather than my Mp3+E17 as amp only).
Id say if id have to give the HE-400 a power requirement id rate them between 650mW and 900mW.
Good to know the minimum requirements you think for 400s.I'll bear it in mind when buying a smaller second amp. I think that table was actually provided by Hifiman during an interview about the line.