The K 250 don't seem to have Sextett drivers, though; not according to the service doc, anyway.
Hard to tell what sort of standard equalization, if any, was aimed at with the 250 and 340 based on the graphs I measured...
There's a picture
here comparing free field and diffuse field. I suppose you could see some evidence of the free field trace in what Tyll measured for rhythmdevils' pair of K 340.
The K 250 as I've measured it fits neither of those.
Here's the raw (i.e. un-diffuse field compensated) graphs for the K 250, K 340 and DT 990 (vintage) as I measured them:
If you look at the DT 990 graph (middle), then compare it to the diffuse field graph I linked to above, the similarity is pretty striking. (Though could be incidental, as my recording setup isn't pro.)
Adda linked a graph (
here) comparing the 240 DF to the 240 M. The graphs are presumably compensated, but I don't know with what curve. My compensated curve for the K 250 isn't actually too far off the DF as it was measured by that person – the DF has a hump around 5 kHz as well. (But again, I've no idea what sort of a recording setup that person had.)
I haven't had a chance to test the K 250 with any other sources yet. And to be honest, I don't really have any.
But I'll have a look at the bass department when I get around to my 250/340 thing.
I've never heard a 240 M, but I do own the Philips SBC 3178, which are a Made in Austria type of thing, and most people seem convinced that they're a rebadged K 260. The K 260, in turn, has been described as a better version of the 240 M. I don't know how close of a rebadge the Philips version is, but they don't sound as good as the 250 anyway. Not bad, but a bit honky.
I'll measure the Philipses tonight; gets us a bit more context.