In contrast, the interior design, of English baroque style, is incredibly rich. The stylistic details and spatial arrangement are patterned after the work of Sir Christopher Wren (1632–1723); particularly his interior design of 1662–1687 at St. Stephen's Church in Walbrook, England.

The building follows the Latin cross type of floor plan. The organ stands in the chancel; the nave and transepts provide seating space for the audience. The walls, over three feet in thickness, contain air spaces which make the building nearly soundproof and also relatively impervious to extreme weather conditions. The floor is laid in marble squares, alternately reddish-brown and gray-in color. The lower walls, to a height of about ten feet, are finished in dark oak paneling; above that are panels of brocade which serve the double purpose of absorbing excessive reverberation and providing a contrasting texture to the plaster walls in which they are placed. The ceiling is an immense Roman barrel vault, executed in plaster with profuse classical detail. The vault appears to rest on a classic entablature, the cornice of which conceals indirect lighting. Roman Corinthian pilasters at the corners complete the classic vocabulary of the design. The hall is about sixty-five feet in height to the center of the vault; forty feet wide in the nave; seventy feet wide at the transepts; and slightly over one hundred feet in length. With a volume of somewhat over 300,000 cubic feet, the reverberation period of the hall, when empty, is about four seconds.

From 1905 to 1909, the organ was rebuilt by the Methuen Organ Company, a financial holding of Searles. Together with another of Searles' holdings, the United States Tubular Bell Company, the Methuen Organ Company occupied a wooden building on the north bank of the Spicket River, originally built as a woolen mill.

Most of the work of rebuilding the Boston Music Hall Organ is thought to have been done by John M. Ingraham (1866–?), an employee of the Methuen Organ Company. The reconstruction left the pipework and case relatively intact; the action and console were completely new. As originally built by Walcker, the organ utilized cone-valve (ventil) windchests with a tracker-pneumatic action. The console was integral with the case, being in the center niche beneath the bust of Johann Sebastian Bach. The new windchests were of the pallet-slider type, actuated by an electropneumatic mechanism from a detached console.

The organ, rebuilt and housed in a magnificent building, was used only for Mr. Searles' private entertainment. The public was not admitted to Serlo Organ Hall during his lifetime.

Edward F. Searles died in Methuen in 1920. Ownership of Serlo Organ Hall was bequeathed to his confidential secretary, Arthur Thomas Walker (1877–1927), as residuary legatee of the Searles will. Upon Mr. Walker's death in Windham, the property was bequeathed to his niece, Ina Cecil McEachran of Detroit. In 1930, Lillian Wightman Andrew (1882–1961), wife of a Methuen and Lawrence banker and businessman, Francis Martin Andrew (1880 - 1967), purchased a large portion of the Walker estate, including the organ hall.

Ernest M. Skinner (1866–1960), one of the most influential American organbuilders, acquired title to the hall and surrounding properties in 1931. During the ensuing years, he presented public performances of such choral works as Brahms' Requiem, the Bach B minor Mass and Handel's Messiah. In addition, recitals were given by such organ virtuosi of the day as Marcel Dupre (1886–1971 ), Lynnwood Farnam (1885–1930) and E. Power Biggs (1906–1977).

In 1936, he established the Ernest M. Skinner and Son Company, with his son Richmond H. Skinner as vice-president. The enterprise occupied the former Methuen Organ Company factory building which was joined to the hall. One of the most significant instruments constructed at this site was the huge organ built in 1937 and 1938 for the National Cathedral in Washington, D.C. (An interesting coincidence: the original architect of the National Cathedral was Henry Vaughan, who also designed the Serlo Organ Hall.) The company flourished for several years, and many fine instruments were built for clients throughout the country. However, Federal restrictions on the utilization of strategic metals due to the Second World War and the accompanying general business decline brought about worsening conditions for the firm. In August 1942, the company transferred all of its assets to Arthur T. Wasserman and Matthew Brown as trustees to protect itself from creditors. A land court decree in May 1943, empowered the Essex Savings Bank of Lawrence to sell the hall and factory building as properties covered by two mortgages: one given by Francis Martin Andrew and Lillian Wightman Andrew, and the other given by Richmond H. Skinner.

The wooden organ factory building was destroyed by a general alarm fire in June 1943. Fortunately, the conflagration was prevented from spreading to the adjoining organ hall building. Essex Savings Bank acquired title to the property at the mortgage foreclosure public auction in July 1943, for $55,000.

In May, 1946, eight area residents organized and filed the necessary papers with the Department of Corporations and Taxation of the Commonwealth of Massachusetts to form a charitable corporation under the name of Methuen Memorial Music Hall, Inc. The primary purpose for which the corporation was formed was to acquire, operate and manage the Serlo Organ Hall as a permanent cultural center.