Stax SR-Lambda Pro headphones
The SRM-1/MK2 drive unit supplies the necessary polarizing voltage for the phones and is also a low-powered class-A transistor amplifier that can simultaneously drive two sets of Pros or other Stax headphones. The SR-Lambda is available in two versions, the more costly being the Pro. Although the two versions look identical, the Pro is designed to be operated at a much higher polarizing voltage; hence the necessity for a special drive unit. It is claimed to provide superior bass range as well as slightly better clarity than the other version.
The drive unit has inputs for Tape Deck, Compact Disc, Tuner, or Preamp outputs, and can thereby serve as the nucleus for a loudspeaker-less audio system. I can think of two very good reasons why someone might want to spend a lot on headphones: first, a frequent need for privacy, for late-night listening, or for blotting out competing noises such as TV or rowdy offspring (that's my situation); second, an active interest in binaural or dummy-head stereo recording.
A binaural recording must be replayed via headphones for proper effect, as no crosstalk between the ears can be tolerated. The resulting soundstage width and sound-source placement can be shockingly realisticsomething everyone owes it to himself to hear. It's a far cry from the inside-the-head imaging obtained from conventional stereo recordings (footnote 1).
The Stax Pros seem to carry the virtues of electrostatic transducers to their ultimate potential: clarity and transient quickness are simply amazing, and combine to produce exceptional resolution of the nuances of music. Gone is the excessive brightness of earlier models. The bass is very tight but only fairly well ex tended. Wear comfort is quite good, though I prefer a snugger fit from the ear cups. (The loose fit may well have been responsible for bass-pressure leakage resulting in a reduction of perceived LF range.)
Now for the bad news. To my ears, the midrange on the SR-Lambdas sounds very laid-back and lacking in presence, implying a broad response suckout in the 2-5kHz region. The warmth region (100300Hz) is also slightly lean. And finally, the highs have a rather transistorish hardness, which this may be the fault of the drive unit rather than the headphone. But since they are inseparable it doesn't matter which is responsible.Dick Olsher
J. Gordon Holt adds some thoughts: I heard the Pros and I didn't much care for them either. The vowel-like midrange colorations heard by individual listeners from headphonesthings like awk, eh, and ih (rhyming with this)do depend on the shape and size of one's external hearing equipment, which is why people often disagree about the sound of headphones that are fairly neutral through the ranges involved.
But the Pros are not neutral. I had the feeling the entire range from 800Hz to 5kHz was depressed, which is exactly what Dick Olsher heard, although he attributed it to a narrower-range dip. The effect was to move everything farther away, which is probably exactly what the designers intended, because headphones that are subjectively flat give the impression that instruments are blaring away right at the ear. Many listeners may prefer the exaggerated distance. But along with that midrange suckout went a pronounced impression of excess midbass and a tizzy high end, which I am sure many people will dislike.
So I must caution prospective buyers to give these a listen before buying. For listening to binaural recordings I would not recommend them, because properly made binaural recordings depend on linearity across the board to re-create the proper distance cues.J. Gordon Holt
Footnote 1: Many people do not get out-of-head frontal imaging from headphones even from binaural recordings, reporting rather that those sounds seem to be inside or behind their head.J. Gordon Holt