TLDR early advanced player using a good instrument that others can play in tune and play flat in my lower/middle range beyond what is lippable, looking for advice.
I'm really struggling to play in tune. It might be two separate issues? There's the usual low notes flat, high notes sharp that you would expect on a Besson. The other issue is that it is just tuned too flat overall even with the tuning slide fully in and a mouthpiece that sticks out a normal amount. These combine to be 40 cents flat on the lowest 3 harmonics (treble/transposed pitches- C, C, G) if I play as relaxed as I like to play. It's slightly sharp on the 5th and sharp but just about manageable on the 6th. It's bad enough that it's easier to tune myinstrument to A than Bb in a cold room. Any conductor I play under picks up on the issue on (Bb treble) mid stave F and G within the first rehearsal even when I'm trying to lip those specific notes up. I'm nowhere near as consistent as I should be either, with it varying day to day but always an issue.
I played baritone for 10 years then moved to euphonium 18 months ago. For details, I went from a be955 + 6BS --> 967 + 6BL 1 year --> 967 + 4AL 6 months. The mouthpiece jump has gone well for other aspects of my playing but probably isn't helping things here?
It hasn't improved in 6 months and I'd like to think I'm otherwise a reasonably competent player given that I play 5-10 hours a week and have 10 years of experience. For a reference, with some effort, I'm able to play a sovereign baritone with the tuning slide fully in within 5 cents whilst looking at a tuner on all notes in the usual range. I would still like a little more room to allow me to play more relaxed, tune when it's cold, or in a sharp ensemble. I have also had issues with other baritones in the past.
Any ideas, or similar experiences?