I can't understand how people can say the 240DF is "more detailed" than the 240M, when as mentioned, they are essentially the same driver just tweaked for diffuse equalization.
The DF indeed has a flatter response, which resulted me in not liking them initially. But then my ears got attuned to the DF tonal presentation, eventually, I liked them more than the M. But I can see some people liking the M more on a first or side-by-side listen. The DF is an "acquired taste". Head-fi listeners here are "connoisseurs" which explains the leanings (I think) to the DF. The merits of the DF flat response come to the fore over extended listening periods (hours at a time). For mixing DF makes sense. For day-to-day listening for an hour at a time, the M probably is more pleasurable.
Also confirming, the DF definitely kicks it up a notch with a good amp (I used to power them with a Wheatfield HA-2) ... They become more dynamic and alive. Being able to easily drive a 600 ohm load makes a big difference with the DF especially (more so than the M in my experience). Why, I don't know, but that was my experience. They are both 600 Ohm, so in theory, should behave the same.
I would say both are "old school" sounding relative to some of the newer headphones out there today. There is nothing necessarily wrong with that. I cherish the old school sound of my preferred K340s. Just as long as you don't pay crazy money for them, they are decent value classic cans. However, I would not invest a lot of $ upgrading them, and instead put the money towards the better New Gen phones.
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