William, who never actually lived in Decatur, was the first Powers to start building commercially. After purchasing farmland in rural Macon and McClean counties, in 1856 he turned his attention to the city, constructing a group of buildings in what became known as Powers Block — the south side of 100 block of East Main. Among those buildings was Powers Hall, which housed Decatur’s first auditorium.

Dr. Rudolphus Rouse's Performance Hall was located at the North West corner of Main and Jefferson Streets, and played host to many theatrical and opera companies. It was called "Main Street" theater after 1902. 

http://collections.carli.illinois.edu/cdm/ref/collection/bra_peoria/id/1

I have found no documentation indicating the name or location of Mark Twain's lecture in Galesburg.  Yet Galesburg does have an appropriate site:

The lecture was held in the new Hardin’s Hall, located on the north side of the 100 block of East First Avenue. Built in 1865 by hardware merchant Chancy Hardin, it was at the time the only public auditorium in Monmouth.

The night Mark Twain ‘vandalized’ Monmouth

I am unable to find a location for this site although I have found references such as "Library Hall and Reading Room in Joy's marble-building on Lake Street".  There was on Lake Street:

In 1863 the Rockford post office was located on S. Main Street what was then the Holland Block. At that time there was no mail delivery and people living on the east side of the river were forced to walk to retrieve their mail. So great was the agitation that several prominent citizens talked of building the post office on West State Street. Finally Horace Brown came forward and said that if he could be assured of the post office he would build a structure to house it. This assurance was given and the construction was begun. When the building was completed Mrs.