Which U.S. president did the drum corps from New Hampshire's Post 25 meet in 1929?

Herbert Hoover. According to the Centennial Celebration page of the post in Newport, "On the way to the American Legion Convention in Louisville, Kentucky, the Drum and Bugle Corps met with President Hoover at the White House."

First chartered in 1919, the post's drum corps had early success after its formation in October 1927; it had already won the state competition for the right to represent New Hampshire at the national convention.

The group and president (Hoover had only been elected the previous November) turned out to have met just a few weeks shy of history. The convention was held at the very end of September and beginning of October; Black Tuesday, the climax of the stock-market crash that started the Great Depression, was Oct. 29.