600 ohm will likely require an amp, but can't you get the 140's in a "studio" model rated at 60 ohms that don't? I picked up a pair of the k240's two weeks ago and I love them. I got the "studio" model and they play just fine on my home stereo's amp. Although I love them they are very revealing - you hear the good and the bad in every recording. Personally I prefer their sound to other headphones I checked out but I can see where some people would find them to be too revealing!
I do most of my listening in headphones now, these big ass Klipsch speakers are going dormant in my living room thanks to these AKG cans...