I like my K240s better than my CD870. I replaced the 870 drivers with SR60 drivers though. So if memory serves me... the 870 has a more pronounced midrange band from ~600-1400Hz. It can make it sound a bit honk-ish at times. .. just a little though, and certainly not enough to make it a bad sounding can. Sub bass extends equally but, theres more of it in the AKG. I like 870 treble though, nice airy. In terms of image and soundstage theyre both equal, but different. The CD870 soundstage is bigger/wider, but theres a distinct "hole" in the middle. Guitar solos sound like theyre coming from a pair of speakers rather than a musician's amp rig on stage. Where as the AKG theres a hint of center stage and some equally good separation., but the image is not as wide as the CD870.
Theyre both good cans for the $$$ IMHO. Never heard the CD1700.
I like my K240s a lot, and have NO troubles recommending it over the MS1, SR80, SR60, and DT770
Garrett