Hi,
True, about the small number of visitors at the Resistance Museum. I first got there in 1989 when it opened up and have been back since then 2 or 3 times. Each time the only visitors I saw were German, one time a German language guided tour, which I didn't know was offered, ....very good to know.
I don't recall seeing any foreign visitors, Americans included.
I would suggest two other sites connected with this history and time period if you want to pursue more in Berlin and its environs.
One is the Ploetzensee Memorial (Gedenkstätte Ploetzensee)... pretty grim, poignant, where the conspirators of July were executed.
The other site deals with the Russo-German War, the Nazi war of extermination on the Eastern Front. This is the German-Russian Museum in Berlin-Karlshorst on Treschow Allee, housed in the Wehrmacht building where the Soviets received the German surrender a day after their surrender in Reims, France. In all the German signed 4 surrenders in 1945.
Take S-3 (Direction : Eckner ) to Karlshorst, the bus at the station goes to the Museum, (Endstation) or you can walk it, I've done both, ca. a 40 mins walk. Very enlightening and revealing in historical portrayal if one is into the Eastern Front in WW2.