^ Me I'm waiting for Complex's expanded comparison review of the SRH840 & SRH940. I'll add to Mad Max's list of review questions..
Bass: which handles drums better? Are 940 lacking bass, or are 840 excessive? Are 940 well defined/crisp?
Mids: which has more 'forward' mids? Quick test- which makes electric guitars more exciting? Puts voices further in front of the other instruments? Are mids smooth? Voices realistic?
Highs: do 940 have a good amount of treble? Listening to a song with lots of cymbals (and decent bitrate) are they ever excessive? Or well balanced with the other frequencies & instruments?
Soundstage: listening to acoustic or classical music with your eyes closed: does one sound like the performance is in a larger room? Is the soundstage natural or artificially created by treble? Test for this is if bass sounds like it is in the middle of the room and treble on the fringes.
Isolation: does one block external sounds better than the other?
Musicality: do 940 sound a little less bland, nasal or closed-in than 840?
How much more insight and detail does the 940 provide versus 840? Speed? How much airier is 940's soundstage? How much more does 940 bring out ambiance and what not? How precise is the soundstage?
Does 940 have a tighter clamp grip? How much sturdier does it feel when you fiddle with it?
Take your time. =]