I will admit that this is copied from a summary of the organ, but it seemed all too perfect and concise:
The Symphony Hall organ, a 4,800-pipe Aeolian-Skinner (Opus 1134) designed by G. Donald Harrison, installed in 1949, and autographed by Albert Schweitzer, is considered one of the finest concert hall organs in the world. It replaced the hall's first organ, built in 1900 by George S. Hutchings of Boston, which was electrically keyed, with 62 ranks of nearly 4,000 pipes set in a chamber 12 feet deep and 40 feet high. The Hutchings organ had fallen out of fashion by the 1940s when lighter, clearer tones became preferred. E. Power Biggs, often a featured organist for the orchestra, lobbied hard for a thinner bass sound and accentuated treble.
The 1949 Aeolian-Skinner reused and modified more than 60% of the existing Hutchings pipes and added 600 new pipes in a Positiv division. The original diapason pipes, 32 feet in length, were reportedly sawed into manageable pieces for disposal in 1948.
In 2003 the organ was thoroughly overhauled by Foley-Baker Inc., reusing its chassis and many pipes, but enclosing the Bombarde and adding to it the long-desired Principal (diapason) pipes, adding a new Solo division, and reworking its chamber for better sound projection.
Æolian-Skinner Organ Company, Inc. — Æolian-Skinner of Boston, Massachusetts was an important American builder of a large number of notable pipe organs from its inception as the Skinner Organ Company in 1901 until its closure in 1972. Key figures were Ernest M. Skinner (1866-1960), Joseph Whiteford, and G. Donald Harrison (1889-1956). The company was formed from the merger of the Skinner Organ Company and the pipe organ division of the Æolian Company in 1932.
Here is a link to photos taken when the organ was re-built in 2003.