There are a whole lot of AKG 240 models... and, apparently, at least some of the older AKG 240 Studio models were 600 Ohms.
There are also several Beyerdynamic models that have 600 Ohm
versions (several models actually come in several different impedances).
Some headphones actually show the impedance on the earpiece somewhere.
You may be wondering
WHY headphones would come in different impedances... and there are a few reasons... with varying degrees of relevance.
First off, when we're talking about dynamic headphones, the physical difference, all else being equal, is that a higher impedance driver has more turns of thinner wire.
The usual result would be that, electrically, the higher impedance driver has both more resistance and more inductance.
While this might suggest that the one with more inductance might roll off the high end more, this isn't necessarily true with real world designs.
In the old days, high-impedance headphones were often preferred because they worked well with transformerless tube headphone amps.
The recommended load for a tube in most configurations is in the range of several thousand ohms.
The purpose of the output transformer in a tube amp is to match the high output impedance of the tubes to the low load impedance of a modern speaker.
Many tube headphone amplifiers have limited output capabilities, and can deliver plenty of voltage, but limited current.
High impedance headphones are a better match for this.
Most modern headphone amplifiers have limited voltage and current capabilities.
And the headphone outputs on devices like smart phones have
VERY limited output capabilities.
Most modern headphones have an impedance between about 20 Ohms and about 50 Ohms, and also relatively high efficiency, so they work well with most amplifiers.
Certain planar designs have low impedance and very low efficiency.... so many headphone amplifiers are unable to deliver enough
CURRENT to drive them well.
Exactly the opposite, high impedance headphones draw very little current.... but many modern headphone amplifiers are unable to deliver enough
VOLTAGE to drive them well.
To be honest, this usually isn't an issue with most efficient modern headphones unless the mismatch is really extreme.
For example, a transformerless tube headphone amplifier is a bad choice for driving low impedance headphones with relatively low efficiency.
And, in the obverse, the output of a smart phone, which delivers very little voltage, will have trouble with high impedance headphones - unless they are very efficient.
So, if there is any doubt, it's best to stick with headphones between 20 Ohms and 50 Ohms, and relatively high efficiency.
Older 600 Ohm headphones may have a somewhat lower efficiency than newer ones (especially if they were made before neodymium magnets were available).
This may make them difficult to drive for headphone amplifiers with limited output voltages.
(The BasX A-100 drives both types of headphones
VERY well.... which makes the choice of which headphones to use less critical.)
Beyond all that, however, because the mechanical characteristics of each are somewhat different, the different impedance versions of the same model headphone tend to sound somewhat different. Therefore, beyond actual drive capabilities and efficiency, many people simply
prefer the sound of one or the other version of a particular model.
Incidentally, the normal common types of taper on a potentiometer are Linear, Logarithmic, and Audio taper.
In general, since our perception of loudness is more or less Logarithmic, you would
never use a Linear taper pot for audio volume control applications.
In general, most audio devices use an Audio taper for their volume control (it's sort of a variation on Logarithmic taper optimized for audio).
Standard Audio taper potentiometers, because of the way they're made, are susceptible to increasing channel mismatch near the bottom of their travel range.
Digital or digitally controlled volume controls are usually programmed to deliver a variation on the Audio taper, and usually avoid channel mismatches quite well.
Don't worry TOO much about the 'oclock' of the VC. Does the amp or ear speakers distort at higher settings? If the headphones distort, you probably don't actually need more power. If the amp gives up first? Play with the jumper settings. This amp should drive
nearly any reasonable set of headphones.
BTW? My reading of the AKG site shows the 240 to be 55 ohms. There are a couple K240 models listed, the MKii and the 'studio'. Who told you 600 ohms or do you have an older model of the same 'name'?
There are 3 major VC types, called 'taper'. This is the relationship between the resistance and the rotation. The 3 are 'Linear' , 'Logrithmic' and 'AntiLogrithmic'. I think Logrithmic is what is called, generally, Audio Taper. You could substiture ANY of the 3 into a circuit and get different rotation to
volume level 'feel' out of them.