I would get the 440s and put the difference towards an amp. An alright amp doesn't cost that much, and these things LOVE power. I have no idea how much they draw, but the better the amplification, the better they sound. I plugged them into a Behringer 4 channel rack mount amp, it's got more than enough power (can drive 8 ohm speakers that are are sensitive enough), anyway, I have broken them in... the 440s already weight a bit more than the average pair, and if they were a 3rd more, I may have fatigue, but at their current design and weight, they are very well made and comfortable.
They are fast, wide sound sound stage that is not like a lot of other headphones, but can still dip low provided you have the power for it. I think midrange and up is going to sound well, but the bass control, as in, how well the headphones can accurately and quickly move about bass transients, this is going to depend on how much power you have to give them. I think there are "voltage" headphones and there are "current" headphones and these fall into the "current" catagory. If the 840s are the 440s, but MORE, then I would seriously think about an amp, even if you are happy with what they give you right out of the ipod. I have a feeling these are sleepers. They can give a lot more than they look. If you pick up a pair, you can certainly see how well made they really are, then have your own opinion of them. I have a fairly good non-os DAC, amp, digital interface, bit-perfect output, and so forth, but I don't think I have pushed the Shures beyond any limitations. This is just the 440s. They do sound hot at first... breaing them in helped, as did dialing the trebel down about two notches on the amp. After that I could have been transported into another land with these. I really like them.
