The E 90 amplifier also share the same plasticky build quality as the headphones. The entire box looks like a DIY build. Koss must be obsessed with minimizing weight when designing the system, because the E 90 is also very light weight. It may be good if the E 90 is a portable amplifier, but for a desktop amplifier, something this light doesn’t inspire confidence. Conveniently, it takes two types of input, RCA and 1/8″ mini jack. So you can hook up both your iPods and your CDP into the E 90. The volume knob, like Stax amplifiers, controls the left and right channel separately and can act as a balance control.
The quality of the E 90 is debatable. Most people will suggest upgrading the amplifier to a fairly affordable Stax amplifier like the SRM-001 Mk2 (which requires you to fabricate a Koss to Stax connector adapter). Better amplifiers like the Woo GES or the HeadAmp KGSS can also be used with the Koss, though the bias voltage of the ESP-950 is slightly higher than a Pro Stax bias. I am writing this review using the standard E 90 amplifier, and all sound quality impressions is based on the E 90 amplifier.
You wouldn’t bear with the plasticky build of the ESP-950 if its not for the sound it is capable of producing. If you like a detailed and analytical headphone such as the AKG K701, the ESP-950 would give you everything you like about the K701 and more. I think finesse is the word I’m looking for to describe the ESP-950. Electrostatics have such a great finesse about their sound, and the ESP-950 is a great example. Effortless is another word that I’d like to use to describe the ESP-950. They are very fast in transients, very transparent and detailed, yet everything sound so effortless. If you have watched Kill Bill, then you must remember the elderly kung fu master Pai Mei in Part 2. Pai Mei represents the ESP-950 very well. Extremely skillfull and effortless.
The ESP-950 sounds very natural, smooth, and musical. It may be revealing and precise, but it is far from boring and analytical. The sound just flows very naturally. The presentation is very easy on the ears, no brightness and no harsh treble often associated with dynamics that tries too hard to sound detailed. Though the ESP-950 is fairly revealing and detailed, it is actually not a detail monster. It is also quite rolled off in the treble and in the bass, though I am sure that these deficiencies can be attributed to the E 90 amplifier. The soundstage is fairly wide, though nothing stellar, but it has a superb separation between instruments. The best thing about the ESP-950 is that it presents music wonderfully. I am listening to some accoustic tracks from James Blunt’s “All The Lost Souls”, and the presentation from the ESP-950 is scary good!
