Altec Lansing's famed 604 was not the first Duplex® coaxial speaker; that honor goes to the company's 1941 model 601, which mounted an HF compression driver onto the back of a 12-inch woofer with a hole cut into the center of the magnet, forming a throat for a small multicell horn in the center of the cone. However, it was Altec Lansing's model 604 coaxial that created a splash that continues to this day. The 604 was based on a 15-inch woofer with a 3-inch voice coil within an Alnico V permanent magnet, combined with a large HF driver and a six-cell horn. The 604 was capable of a then-impressive 30 watts of power handling, but due to its high efficiency, wide bandwidth and point-source imaging, it was soon adopted as a standard monitor in studios.
Altec continued improving the 604 through the years, and the drivers have been used in numerous variants, either in stock “utility” cabinets, custom enclosures, modified third-party designs (Big Reds) or with an alternate crossover such as Doug Sax's Mastering Labs model. The affinity for the 604 may have waned in the studio, but it later regained popularity as the basis for UREI's 811/813/815 monitors.