AKG K240 Monitor Studio Headphones, from Austria.

There are at least 5 versions of the K240, and these are the Monitor version (2058Z0015: C).   This Early model appears to be the third sold by AKG, and were supplied with a 1/4" TRS connector.   AKGs service documentation indicates that they will continue to provide service, support and parts for these headphones (see below).   Gotta love these guys.

We could not find complete specifications for this version.   The latest version is of a lower impedance (55Ω) and has similar construction.  At the bottom of this page there is a 2007 review of the K240M (Monitor) and the K240S (Studio).

From AKG:

Parts for the K 240 Studio will be sold as long as our stock lasts. For later repairs we will exchange the headphones against K 240 Studio II at special rates but only in case that parts are not available. In case of a capsule defect in very old K 240's of variation 01 and 02 it may be necessary to change both capsules. Disks 22 with passive radiators are no longer available. All disks 22 are delivered with covers 33 already attached. Lid 12 contains the Mini XLR socket 38.

Features:

....The AKG K-240 Monitor is the classic studio monitor model, with an impedance of 600 Ohms (sensitivity 88 dB SPL/mW) and a dynamic range of 15-20000 Hz, according to AKG.

K-240 Studio has a nominal impedance of 55 Ohms at 1 KHz, a sensitivity of 91 dB/mW, 104 dB/V, the frequency range is 15-25000 Hz, according to AKG. Which it may well be. Though the 25000 Hz won't be anywhere near 0 dB, rather it just indicates the headphones' ability to reproduce an audible tone at 25 KHz. 15 Hz, on the other hand... Yes, why not. Quite so. Within -3 dB.

First thing first, the K-240 Studio are not real monitor/studio headphones. Those are a listener's headphones. They are built exquisitely, they sound powerful, and manage a certain bass exaggeration without drowning everything in a single-point bass range. Bassy, but not a boombox 100 Hz rumble. Defined bass. A precise term would be "EQ shifted towards emphasis on lower frequencies".

An emphasis on low-mid frequencies may sound warmer (to most non-musician/engineer folks anyway, the mixing people require clean response, frequency graph as straight as possible, with adjustments for DF EQ in the case of headphones), but it also tends to be fatiguing. The K-240 Studio, in spite of weighing less than the K-240 Monitor, are nevertheless more tiring, unless equalised so they don't "kick the tympanic membranes".

Now, the comparison.

Where the K-240 Monitor shows the entire body of sound, flowing and perceptible, the K-240 Studio manages to capture somewhat more detail (the Monitor can be a tiny bit murky at times, especially with multi-frequency, multi-layer synthesised strings and other complex instruments, by comparison with the Studio).

The K-240 Monitor sound more truthful, even "sweeter" as they don't attempt to translate the pressure of a drum's bass, just the music, even if that means the K-240 gets overdriven more easily. The 55-Ohm resistance really isn't that helpful - the 600 Ohms of the older model provide a more laid-back yet more precise reproduction.

The K-240 Monitor is much more sensitive to lower-frequency material. Its real -3 dB cutoff must be around 40-60 Hz (or higher, depending on how saturated the low-frequency range is in the mix - the Monitor can dislike "thick" bass). The K-240 Studio manages to play back all lower-frequency stuff without being bothered about current much (volume can be raised to deafening levels easily), but it has a bit of a certain plasticky stiffness to it characteristic of dynamic headphones. Once again, it's just a bit of stiffness, and it is usually "listened through".

The nice bit about most current K-240 headphones is they're equalisable. So if the K-240 Studio are to be considered for home studio use, they ought to have a proper equalisation. A U-drop in the lower frequencies from bottom to 440 Hz (311 at -3), with 30-60 Hz depressed just a bit, to retain the drum kicks' punch. A gentle +2 dB push in the 7500 Hz+ region (skull bone resonation frequency) also helped, AKG must've been overly cautious with dampening response at that range.

55-Ohm impedance does mean the K-240 Studio are pretty driveable. A Sony portable CD player (D-NE500) could drive them with the meager one AA battery, to listenable level. Detail wasn't the same as with a (9V) headphone amplifier, but it certainly was listenable to.

Overall, strangely enough, those are the headphones that teenagers would love. Or maybe someone with a love for Megadeth, Judas Priest (for those older), and so on.

The K-240 Studio have been called "the groovaliser". Sometimes they do put out a lot of bass, especially in the drumkit bass kicks, but they're very warm and a revelation for anyone coming from regular consumer headphones. Sonically they're similar to Sennheiser HD-25-1, which usually cost twice as much.

They certainly are rock-optimised. They also play piano great - George Gershwin's piano rolls album inspires dancing. Classical music sounds full, rich with the K-240 Studio.   And they certainly are portable. They benefit greatly from a headphone amplifier, but digital audio players should be capable of driving them to listening (and sound-isolating) loudness on their own. Of course the detail won't be the same as with a headphone amplifier.

The K-240S aren't closed, so they don't isolate sound as well as real sealed headphones. The semi-closed design and powerful bass make them excellent sound-isolating street (and train) headphones. At listening level on the street, almost nothing comes through except louder noises (whistles, sirens, etc.) and maybe the occasional boombox. They have also properly made supermarket radio non-existent. The very low bass response means any low-frequency sound is transmitted properly. Those would be excellent headphones for a session drummer (which seems to be AKG's intent, as the K-240 render drums very realistically).

But for regular mixing and sound engineering work, the K-240 Monitor is still a better choice, also for listening when there's an amplifier that can drive them (even some cheaper CMoy headphone amplifiers can drive the K-240 Monitor, though not as loud as the Studio). The K-240 Monitor are more even-tempered and possibly render an image closer to what it might sound like on a hi-fi home stereo.

The K-240 Studio, on the other hand, can be a great, enjoyable walk-and-listen (or general-purpose, or drummer's) pair of headphones. When equalised.