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Try the smaller mouthpiece. It worked for me years ago when I was playing an old American Eb (never new the brand but always thought it was an old Conn or York).


Will

In those days, B&H made their instruments 'in tune' with the mouthpieces that they supplied with the horn. The only problem was that the mouthpieces were Kosikups! As soon as you wanted to use a decent mouthpiece (UK players seems to favour Wick 2 & 3 or Bach 24AW) the pitch went south.
The solution was to have the back bow shortened by 38mm each side. I bought a 19" bell Imperial brand new in 1980 and it was annoying that the first thing I had to do was have this bow shortened (wrecking the lacquer) because B&H refused to do it in the factory. In his excellent book, The Tuba Family, Cliff Bevan writes about it more extensively (see page 168).
Thankfully with the introduction of the Sovereign Eb, this bow was shortened in production rather than after.
A new leadpipe should help upper register intonation, I would suggest a Sovereign 981 leadpipe rather than a 982 one.