After days of listening to these and comparing them mainly against my Shures (seems like a natural competitor):
XPT100/HM5 has slightly more bass, but SRH840 bass is much more textured/detailed and EQ-friendly. SRH840 seems to extend lower before any audible mud.
XPT100/HM5 has smoother, lusher mids, great for electric guitar, but vocals occasionally sound distant and slightly less detailed than SRH840 mids.
XPT100/HM5 has watered down treble. Non-fatiguing listening experience, but fake-sounding cymbals. SRH840 treble is painfully honest and never flatters an overly bright mix.
So in summary, both of these headphones are fairly "neutral" for their price range, but in totally different ways. The XPT100 is neutral in a pleasant, balanced way and a real all-arounder. The SRH840 is neutral in an unflattering, transparent, high-resolution way.
Build Quality: SRH840.
Recreational listening: Tie: SRH840 for 'wow factor' and XPT100 for extended, fatiguing sessions.
Analytical/studio applications: SRH840 by far. Dangerously honest headphones.
Edited by machoboy - 9/16/12 at 2:30pm