I've used HD650 for over 2 years. I've listened to them amped and unamped. Without proper amping they won't realize their wonderful potential and will sound far worse than amped (and unamped) SRH-840 despite the difference in sound signature. I don't wont to force anyone, it's just my opinion.
IE8 are not in this contest
let's summarize: ampedHD650 > unamed AND amped SRH-840 > unamped HD650
My personal taste is in favour of the 840s but I rate the senns as better headphones it terms of detail and timbre which is important to many audiophiles