Please remember that even though HE-6 is the worlds current production least sensitive headphone, it still reaches 90dB with 20 milliwatt.
This means that unless you listen at extreme ear damaging SPLs you will do with much less than 7 Watts. My Master-6 amp is 7 Watts and Soloist is about 1.7 Watts in 50 Ohm. The latter is in no way lacking bass power or producing edgy or sharp treble. Read the review at Headfinia: he actually says the Soloist drives HE-6 better than Dark Star, which noone blames to be under powering the HE-6. Also, preproman's First Watt (current source) speaker amp, that he finds extremely good with HE-6, is also less than about 2 Watts in 50 Ohm.
This implies to me that it is not about wattage, as long as you have a properly dimensioned power supply.
Quality of the amp is far more important than extreme wattage IMO. HE-6 is phenomenally transparent and resolving and this will be compromised with a mid fi, high power amp.