With a good set of headphones and a little judicious equalization you can get close enough to flat for human ears. Again... look at the thresholds of human perception. Human sensitivity to frequency response imbalances is between .5 and 1 dB with tones. But under music, it's more like 3 dB. The narrower the Q of the imbalance, the less important it is. Knowing this, it's not at all difficult to take a good set on headphones and tweak them to be audibly flat. People get way too hung up on splitting the atom. Sound Science is made up of everyday principles not nuclear fission. It isn't hard to get good sound for a reasonable price. It's a lot harder to do that by following vague reviews and constantly upgrading equipment that doesn't need upgrading.