Having both the K-240DF and Monitor and Studio... They're nothing alike. The DF are co-o-old. And this is coming from other monitor headphones (ATH-M40). That is more or less all right really, they can get nice with the right type of music, but, there's that other quirk which is rather putting off. Everything in the DF sounds as if it were floating somewhere in the middle of space with no walls. Yes, this is how monitor speakers in an anechoic room might sound (or might've sounded, the new ones typically have a bit more presence in the very high frequencies, 10-20 KHz). That's my gripe with them. It can be a bonus, suppose if you're listening to classical music off CDs or MP3 files (gasp), then the DF will actually kill off the nastiest, harshest high frequencies above 11 KHz (that's low-res sampling for ya, 44100/11025=4 coordinates/cycle). Same about jazz, they might have a coldish tint, but they'll simply remove the harshest parts and present the more realistic midrange/low treble. Which is why many Head-Fi owners might love them. Maybe with the right "warmish" amp they're nicer, but here, straight out of a sound interface or mixer (or even tape deck), they're, well, clinical.
But with decent high-res sampling formats and vinyl and tape the DF are, well, cold. They're pretty useful as a tool, for editing, say. Some folks still swear by them as THE headphones that can replace monitor speakers (if you've ever mixed music, you'll know this is sort of a holy grail of pro headphones' owners). Listening for pleasure though? Mmm no, again, that soundstage out-in-the-middle-of-nowhere-with-no-walls is just too disembodied for me. It might work for some electronical music as a "fantastic space arena" maybe...
Frankly, of the three, the Studio and the Monitor are nicest. Recabled Monitor can give a realistic impression of anything (e. g. there's that part in Jarre's "Chants Magnetiques" that gives the impression of floating on top of a magnetic wave, to use an example from the era when K-240M were produced). Studio actually are the ones with the worst sound quality of all (in spite, or maybe because of the largest diaphragms). They're quicker, but they're also coarser than the Monitor (and DF) properly driven and recabled (older AKG headphones, like the Sextett and K-141 Mk. 1, can be stunning recabled, it's a 3-D kind of sound).
So... If there's anything to recommend to someone who's looking for a K-240DF replacement, it's either another DF-equalised headset (DT990?), or an AKG sibling like the Sextett or K-141 Mk. 1 (silver). These won't disappoint, the K-141/K-240 Sextett are warmer than the DF while having a similar spacey character.
