Putnam—unhappy with the sound of the Altec 604 monitors in his United  Western Studios—worked with UREI’s Dean Austin and Dennis Fink on ways  to improve the 604. They replaced Altec’s multicell horn with a wider  dispersion design and added a 15-inch Eminence woofer to boost LF  output. Ed Long applied his Time-Align™ crossover techniques to achieve  time- coherent, true point-source performance.             	Engineers and producers mixing on the system  were so enthusiastic about its sound that UREI started producing the  monitors as a commercial product, with the first UREI 813 debuting in  1977. Typically soffit-mounted, these large, double-15 monitors were  ideal for the larger, higher-SPL control rooms of the time.    
                     UREI founder Bill Putnam  Sr. tests a very early 813 prototype. 
                Two years later, Altec replaced its Alnico  604-8H with a ferrite model, requiring modifications to the 813 design,  including a foam diffraction buffer, crossover mods and small Helmholtz  resonators in the horn flare. This 1979 model was the 813A, followed by  the 815A (a 604, plus two extra woofers) and the single-driver 811A, but  the 813A was far more popular.    
           	Financial and QC problems at Altec led UREI to  find a new driver source, now mating PAS coaxial 15 to a JBL 2425  compression driver. The new 813B version debuted in 1983. Later that  year, Putnam sold the business to Harman, with UREI becoming a division  of JBL Professional; the 813C, a new model with all JBL drivers,  launched in 1984. But in its various incarnations, the UREI 813 was the  most successful large-format studio monitor ever made.    
takodje koristan link : 
http://www.audioheritage.org/vbulletin/showthread.php?21472-Great-Plains-Audio-Repair-of-Urei-801