Ymer: The K240M (M = Monitor) and K240DF (DF = diffuse field equalized) are old early 80s constructions with very high impedance. The DF was tailored for studio purposes and is very neutral, but also rather lifeless. The M is a bit more musical, but still not as groovy and extended as the K240S (S = Studio), which is a recent construction with the new low impedance Varimotion XXL drivers - and it also features a detachable cable (2,5 mm micro plug in the first batch, now mini XLR). To my ears, it's a very wothy successor for the older K240 models. Whether it's better than the DT531 is more difficult to say. Personally, I prefer the K240S sonically (comfort-wise the DT531 is better for long term listening due to its velvet earpads), but that probably boils down to individual taste and system synergy.
Greetings from Munich!
Manfred / lini