Thanks, everyone, for the replies. I truly appreciate all of your insights into this issue.
While I am always willing to blame myself for most things, musically and otherwise, in this case, I really don't believe it's just a matter of "learning to play the horn," or that it takes more air, or is more freely blowing, or whatever else. I truly believe the length of the Adams pipe from leadpipe to bell rim is just a bit longer than most other brands. The comments offered seem to back up that assessment, and (especially) the fact that they have a shorter MTS that they offer to sell you. There just wouldn't be a need to do that otherwise, in my opinion.
That having been said, I truly hope that I am wrong! I hope that as I play this thing, I make the necessary adjustments, and the pitch comes up. I am not optimistic that will happen though. I waited until I had been playing on it every day for almost two months to even bring this up, because I thought it might be me.
I am not an "equipment guy." I am 45 years old, and although I get paid the play the trombone (or at least I used to, before Covid) regularly, I am a euphonium player by nature, and it's still the musical situation that I am most comfortable in. This Adams is only the third euphonium I've owned, after playing a YEP321 until college, and then my Besson New Standard from 1997 until two months ago. It IS quite possible that I'm just an old dog trying to learn the new tricks of playing a modern instrument, and that horns have changed so much since 1997 that I just need more time to get used to it. Again, that's not how it feels to me, but I could definitely be wrong about that!
Doug, I don't suppose you're looking to get rid of your shorter MTS, are you?