I still have one of these...although, mine is in a charcoal-colored box, rather than the silver one illustrated. It worked like a champ at what it purported to do, which was to correct for the angle at which the headphone (oops! earspeaker!) poured the sound into your head. I say it worked, because of the extra space in the soundstage that resulted...the illusion of an extra foot of distance in your head was niiice, not unlike Headroom's crossfeed, but, in my head at least, more effective (YMMV). Also. on the couple of binaural CDs I have, there was a real but hard-to-describe increment of reality.
There may also have been some equalization specific to the Lambda Pro - it seemed to even out the frequency response, in the treble (smoother) and bass (a slight peak was erased). To my ears, the Lambda Pros had an overbrightness without the ED-1, and not with it.
You may notice I'm speaking in the past tense. My Lambda Pro headset died in its left channel, a few years ago...at that time, the Stax corporation was apparently defunct. They have since resurrected, I guess, but I still don't know where to have the repair made in this country. So I plugged my Stax SRX mk. III head-uh, earspeakers into the Stax amp, and the sound is sweet and keeps me comfortable...but, the ED-1 makes little difference to these. Either the (hypothetical?) frequency response equalization - which Mk III definitely doesn't need - made the sound a little wrong; or the Mk III resolving power is just too far shy of the Lambda Pro. In either case, the ED-1 became redundant, and I took it out of the system.
