Both have very different presentations and signatures. They sound nothing alike.
The K702 having a pretty bright signature with the treble being smooth on good material and downright strident on modern music. The midrange is slightly recessed but detailed and resolving and seems to make guitars and acoustic guitars stand out more than vocals.The bass is very tight and controlled but does not offer much in terms of impact or deep bass as it rolls off early. The K702 soundstage is very wide but does not have much height or depth. The K702 has very nice transients and nice attack, but not fantastic or top notch. Now the K702 does require a bit more power than it's impedance would suggest. I nice solid sate amp with about a voltage swing of 20V to 30V would be just fine for it.
Now the K240 is the very opposite. It have a smoother and I'd say darker signature. Dark might not be appropriate but I felt the K240 sounded very similar to a HD600 or even a HD650. The treble is smooth and not that articulated with not much extension (rolls off) and there is no peaks. The midrange is a ton fuller than the K702's but not nearly as resolving or detailed. I'd say it's a tubey, slurred type of midrange where it emphasizes voices and guitars equally. The bass is entirely different from the K702. It's punchy yet not extended nor very tight. It's can be somewhat flabby or ill defined on bass heavy music but anything with less bass sounds great. It's just that if you pump something bass heavy, such as rap, the bass becomes sloppy. The soundstage is quite small and doesn't offer anything special in terms of depth, width, or height. I will say it has nice separation of instruments but not great as they can blend together. As for powering it you won't need much. Even a simple Bravo amp would be more than enough for it.
Hope this little comparison helps you out a bit
