or the k400/500's be another great choice to look at. they all can provide an expansive soundstage(if intended by recording. not artificial expansion) and some of the best midranges around. the only other headphones out for accuracy is audiometric headphones. the beyer dt48A is still produced today by beyerdynamics since the 1930's(didn't hit the consumer market till 1950) under special order. you can find some at B&H in stock still. they are well praised for their transparency and one of the most detailed midranges around. the other one that is like the 240DF's that's still in production is the beyer DT48S. they are diffused Field equalized like the 240DF so they provide the same precise accuracy for studio recording. much easier to drive as well compared to the 240DF's. if your looking for extra deep bass extension flat down to 20hz the pioneer monitor 10's are something to look at as well. they are every bit like the 240DF in accuracy but can extend lower and higher. they can reach 20hz no sweat if you seriously need that type of sub-bass and they can easily peak up to 20khz(yes i can still hear this tone. i like to take care of my hearing).
let you know i'm actually taking a risk here letting you know this...well cause the headphones i mentioned are one of the most best kept secrets around here and no one is willing to share their secrets with others.