The second area in which the WES excels is tonal purity. Almost all headphones have some frequency response errors that lead mid- and upper-range instruments to sound a little distorted, especially on certain dynamic passages. The SR-009s reduce these distortions significantly when compared to many headphones. As a result, you have more opportunity to listen for a more subtle effect: the tonal continuousness and lack of grain in the signal. The Woo delivers a sense of purity—which is to say smoothness with detail—that is at a benchmark level in our experience.
Perhaps related to this tonal purity, the noise floor of the WES is impressively low, which means that another signature aspect of the Stax headphones is well handled: small signal accuracy. This matters to spatial presentation because small signals (for example, the decay of a string after it has been plucked) are the sort of low-level details that enable us to hear the acoustics of the venue where a recording was made. Likewise, the overtones of an instrument are the source of its color, but are always reduced in level from the fundamental tone. Here the Stax/Woo combination does a state-of-the-art job of revealing low-level detail in a natural way. You can hear the space in which the recording was made (though, as with almost all headphones, it isn’t presented as if on a stage in front of you).
The Stax headphones are also exceptional dynamically, and the Woo complements this capability nicely. The attack of strings or percussion is accurately rendered, without annoying overshoot in most cases. In addition, dynamic signals settle down quickly, which allows you to hear the decay of notes, as mentioned above. This sense of quick dynamics without accompanying edge or sizzle generally does a superb job of letting you enjoy the music.
Sometimes this breakdown of sound into component parts doesn’t give you a good feel for the sound character on offer, so here is another point of view. The big thing you need to know is that the Stax/Woo combination produces a subtle but meaningful qualitative shift in the presentation of music. Sure, the qualitative shift here could be described by saying that the SR-009/Woo system is more transparent than other headphone/amplifier combos are. Or you could say the Woo-based system is lower in distortion. You might also say the Woo/Stax pair is more dynamically accurate. We’d sign up for all three. But, a point we’re trying to draw out for you is that the qualitative achievement here is one of those across-the-board shifts that affects almost everything rather than provide small, piecemeal improvement in just one or two narrow areas. Because this headphone system removes distortions and colorations that we are used to hearing, it sounds more natural (it isn’t the result of an impressive but unnatural effect confined to one area). It is this across-the-board aspect of the improvements on offer that makes Woo’s (and Stax’s) achievement so meaningful.
But with that said, we do need to mention one concern about the Stax/Woo system. When playing some relatively simple music, the Stax/Woo combination can sound somewhat stressed or shouty on vocals, especially female vocals. This suggests a frequency response elevation in the mid-range. Whether this is a Stax artifact or something created by the WES is not easy to say. This effect is less noticeable with other amplifiers, but that may simply mean that the WES is revealing—but making no attempt to compensate for—a small sonic error inherent to the SR-009s.
Philosophically, we note that all the electrostatic amp manufacturers in this top tier category seem to be aiming to build amps that could conceivably be used with any type or model of electrostatic headphone, rather than creating amps specifically matched to (or optimized for) the SR-009. In that sense, we think Woo’s approach has been to pursue accuracy and neutrality first, and then to the sonic “chips,” so to speak, fall where they may.
Given that the WES is quite good but not perfect, you will want to consider how the WES compares to other electrostatic headphone amps. While we haven’t completed our work on this, we would say that the Woo majors in clarity and dynamics, the Head Amp Blue Hawaii SE majors in harmonic richness and warmth, and the Cavalli Liquid Lightning falls somewhere in the middle.