I've recently listened to the Philips X2 and AKG K7XX... and just bought the HE-400S. I can also confirm that all three have a mid-/low-bass boost. My comparison/observations:
X2: powerful, engaging, full sound (strength: build quality, overall versatility with movies/all genres of music)
K7XX: airy, clean, lean, detailed sound; but I think the boost around 2-3 kHz is a bit too much (strength: gaming, large ear-cups, feel light on the head)
HE-400S: most-realistic sound-stage of the three (especially with orchestral music): superior center image, spacious, with superior instrument placement/distinction (strength: the best of the three for music enjoyment - although it does have a darker sound than I'm used to)
Also, I noticed that the X2 and K7XX sound-stages were too wide for me (extreme L-R), without a proper center image - especially noticeable with orchestral music. I'm glad that I discovered the HE-400S... my first HIFIMAN headphone!