I'm using the stock Chinese tubes with my EF-2A. However, I do guess that one could get into some tube rolling if a desired change in the sound was needed. But, the stock tubes do well with my Fostex, so I don't want to use another set at the moment.
I'm not trying to imply that the EF-2A doesn't mix with the HE-400 in a horrible way. I am implying that while HifiMAN has coined these headphones as efficient orthodynamic headphones - which they are when compared to the others in their lineup, I think the HE-400 really reaches its potential with a more gutsy amp and source plugged into it. The EF-2A sounds good with the HE-400s and my iPod classic with a LOD cable. However, when connected to an amp or receiver with speaker taps ... is where the fun really starts. Also, don't poo-poo the HE-400s because of my response and experience. Believe me, I do like mine and hopefully, will like the new pair that HifiMAN sends me as replacements in a week or so.
I believe almost any headphone will sound "good" with nearly any amp. However, in order for the headphone itself to reach its best potential, then the source and amp have to be considered for proper "great" sound.