I have a 1941 Boosey & Co. Imperial that was horribly flat for me - more than 50 cents. Couldn't play the horn at all, so it sat in my closet generally unplayed from 1984 until I turned it over to Lee Stofer in 2014, who did just as Dave described - cut the tuning slide legs, then rebuilt several of the valve tuning slides as some of them had partially rotted away. Then he cut correspondingly on those valve tuning slide "legs". The end result was I have a spot-on middle Bb when the horn is warm with the tuning slide pulled about an inch.
I'm no technician, so I don't know the detail beyond that, but I do know it's not a job for the faint of heart. Cutting a horn is rather permanent, seems to me, and Lee's estimates turned out right on target.
U.S. Army, Retired (built mid-1950s)
Adams E2 Euph (built 2017)
Boosey & Co. Imperial Euph (built 1941)
Edwards B454 Bass Trombone (built 2012)
Boosey & Hawkes Imperial Eb tuba (built 1958)
Kanstul 33-T lBBb tuba (built 2010)