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There was very little indication of the different Sextett models (EP/MP/LP) until a few years ago. Also, no real indication of the K 250 at all until this thread, and there's still very little on the K 241 as well. Stuff gets found out about only when it gets found out about, and sometimes that's tomorrow rather than yesterday.
I'm amping them with an integrated amp, i.e. the Auzen Bravura, which is using the LME49720NA opamp. There's better amps to be sure – though be careful about throwing out "hard to drive" in this case, because ideally you'll have compared the DF from a good amp vs. the DF from my amp before you go any further with that thought.
There is quite a bit DF info spread out around the web, people with different variations of DF's seem to have the same impressions, if the pads are not flat and the screens are in good condition.
I have driven my DF's off a H/K PM665 (not so good bass extension, very controlled, not very detailed, poor soundstage, grainy) a Audiotrak Prodigy 7.1 HiFi (dull 'inside your head' kind of sound, rolled off at both ends, muffled, bright) a LD mkV (bright sounding, sibilant with some recordings, a little grainy, wide soundstage but lacks depth, piercing at louder volumes) a AudioTrack Podigy Cube (dull 'inside your head' kind of sound, rolled off at both ends, muffled, very limited volume) a Echo AudioFire2 (good detail, flat, good soundstage/imaging, very limited limited volume, could have more extended bass) and an Aragon 18k (tightly controlled, flat, good depth and width, great detail, good extension at both ends, sounds more like speakers then headphones).
The way you describe your DF's, makes me think that they might need new screening and/or a better amp, a soundcard won't drive DF's, not by a longshot, a good place to start is a vintage receiver, I'm sure that would be the case for other 600 Ohm AKG's as well.
The reason I say the DF's a hard to drive, is because I know that is the case, I have driven mine off so many different things.