Yama told me in an email that all the new Lambdas have the same driver. He said that he felt the SR-407 was the best bang-for-the-buck in the line, and that the SR-507 was, by a small but audible margin, the best. Being an audiophile, I of course had to buy the SR-507. I love it. I have read numerous posts by VisualGuy saying how the 507 isn't very good, but that has not been my experience. It is better, clearly better, than my HD-600, HD-650, AKG K701, Grado SR-80, and Fostex T-40 RP-II (ortho). I have large ears, and I find it very light and comfortable. I use it with a Stax tube amp. In my experience, it is the first headphone that made me really, truly enjoy headphone listening. I also find that the sound is very similar to my Quad ESL-989s with both Pass Aleph monos and BAT VK-60 monos (tube). In my book, that's not too shabby. That is my experience, for what it's worth. YMMV. Good luck.