So, here's my 507 impression and pictures... first my brief impression after 1 hour of listening.
I compared it with my SR-303 using a SRM-1. First thing you notice is sound signature is totally different. Sounds closer to an Omega with a rich, slightly dark sound. Coming from the 303, the 507 initially sounds heavy and slow. That's just because the notes have so much more body and weight. In fact the 507 is just as quick as the previous generation lambda. There is ample bass. This is a welcome change from the 303, which has rolled-off bass. The 507's low end is super tight and controlled. The only better bass I've heard is from a LCD-2 powered by a WA5. In my mind the bass is a bit too strong to be realistic, but I enjoy ever so slightly emphasized bass and know many others prefer the same. If you thought electrostats sounded thin and lifeless, this headphone will change your mind. Treble is less emphasized than the 303 and cymbals sound more realistic and less blurry. The 507 has better tonal accuracy than the 303, but it sounds just a bit too dark to be realistic, much like a HD-650. In my mind there are more tonally accurate headphones such as the T1 and HE-5. Overall, this is a massive improvement on the 303 and corrects the weaknesses of the previous generation lambdas. The sound signature is a bit on the dark heavy side, but this actually fits well with the quickness of electrostats. It's strange to hear a headphone that gives the notes lots of weight but is also incredibly quick. Photos!