We do not listen to a diffuse field or a free field sound under these circumstances. It's essentially small room stereo acoustics. This blog has some headphone graphs compared to in room calibrated stereo graphs(audyssey and audyssey with dynamic volume). It's interesting to see what some of the more neutral graphed headphones are: http://dtmblabber.blogspot.com/2013/07/headphone-frequency-response-repository.html
None of them would appear to be utterly neutral.