Interesting timing... I just got home with my new K240 Studios and have been comparing them with my HD650s.
Initial impressions using my CKK3 and PPX3 amps and some Radiohead, Massive Attack, Beck, and Wilco.
The tonal balance of the two sets of phones is actually pretty similar. The mids in the HD650s are a little more forward and tighter sounding, and the bass is a bit tighter and deeper. I was expecting the K240s to be bassier than they are. The bass is definitely there and it sounds pretty good, but it does not shake my brains. The K240s seem to have somewhat uneven treble, and roll off more quickly at the upper end than the HD650s.
The K240s are surprisingly difficult to drive for 55 ohm phones. They must be pretty inefficient. My iPod could only get them up to a moderate level with the volume at 100%, and they sounded pretty terrible that way. Both amps drive them wonderfully, however.
So far I think the K240s are very nice sounding for a $70 pair of cans, but the HD650s sound better in all respects. (As they should at 4-5x the price!) I've only had an hour with them, though, so the opinions stated in this post are subject to change.