The Stax SR-Sigma Pro is a high-quality electrostatic headphone that was first introduced in 1977, making it one of Stax's earlier models and a significant part of the brand's history. The Sigma series represented a departure from previous designs, being the first to feature a true 'earspeaker' configuration. This model was eventually updated to accommodate Pro bias specifications in 1987, utilizing advanced materials such as the PC-OCC cable and 1um drivers. The housing design is notably deep, measuring around 5cm, which ensures that users' ears do not come into contact with the drivers, enhancing comfort for long listening sessions. With an impedance of 140k Ohms and measured sensitivity of 94dB/100V, these headphones are relatively insensitive, requiring a powerful amplifier to achieve optimal sound levels. The overall design is over-ear and open-back, contributing to a spacious soundstage that is often compared favorably to other high-end headphones, such as the Sennheiser HD800. Sound characteristics include a dark tonal balance, with thin and anemic bass below 100Hz but increased upper-bass presence. The midrange is lush and warm, particularly in the lower to mid-midrange frequencies, but there is a noted recession in the upper mids and treble, contributing to a unique atmospheric quality. Users often report that the Sigma Pro headphones deliver a very vivid soundstage while requiring a significant amount of power to drive them effectively. Average used prices typically range from $450 to $800, reflecting their status in the market as a classic model with unique auditory characteristics.
