Both of them are completely different in every way. The K240 Studio is meant as a studio monitor and has a balanced, close to neutral sound, but does have a very slight bass boost. Strangely the headphone is more bassy out of the box and gets reduced somehow after a week or so (to my ears, but maybe not. Felt like it). The K240 has some good mids, a decent amount of bass, but slightly rolled off highs. The Sextett seems to have more treble, but it's a pain to drive.
The 750DJ based on my first 15 minutes seems to be very bass heavy. My pair straight out of the box was too bass heavy and had very harsh treble. I disliked them so much that I didn't even give them a chance to burn in. Many people here love them, but not me. I prefer a more neutral signature.
If you are interested in the K240 MK II, I'd suggest the K240 Studio instead. It costs less and doesn't include the two cables and two sets of earpads. If you can get the K240 MK II for only $20-$30 more, it may be worth it. I think the Studio version has a user replaceable cable. Not sure if the MKII does.
The K240 Studio is one of my favorite headphones up there with the DJ100, SRH-840 and a few others. It's even quite good for gaming for it's cheap price. I got mine as an open box for just $70!
The Beyer Dynamic velour and pleather pads also fit them! Right now I'm using the Beyer Dynamic velour pads on mine and replaced the default foam covering the driver. It does help the mids a bit.
BTW the K240 Studio really needs an amp. You can drive it from an Ipod Touch, but the sound is only "good" and you have to max it. I don't suggest it without an amp. The E5 is enough to make it worth getting though. It's definitely not as power hungry as the K601/K702.
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Have any of you had experience with the AKG K240MKII or any AKG headphones? How would these compare to the SRH750DJ's?