I have recently purchased a K400 (EP). It arrived this morning. It's in new condition and very rare to find such a gem in this condition. I have been listening to the K400 and comparing it with K7XX today. One word: K400 is better. Better in every aspect (except soundstage width which K7XX is very tiny bit wider; but depth, width and height is more balanced on K400 and thus K400 sounds more holographic). And I can't believe the feather weight and comfort of this headphone. It feels lighter and more comfortable than any other full sized headphone that I have (pls. check my profile). K400 also delivers the most "reference type" of sound in my collection. It is my benchmark to compare every other headphone from now on. For pure listening enjoyment other AKG's in my collection may deliver something above K400, but, man this should be the sound for the HiFi seeker. I mean ridicilous, I paid only 50 USD for this gem (second hand but unused condition). Price/performance ratio is unbeleivable!
PS: This HP one is 120OHM but needs power, I am driving them with a Darkvoice TH336SE (Psvane CV181 T and RCA 6AS7G JAN tubes).