bloke wrote:Have asked another "reliable" player (like, maybe a tuba player in the National Symphony, etc.

) blow you horn in front of a tuner...
No. But I should. I have taken a few lessons with Gil Corella, but not with the big instrument, and not to diagnose a problem that could be the instrument. (Also it's been well over a year.) He has absolutely confirmed Roger's formula that fast air = higher pitch.
...and, like Sean says, with the instrument (truly) warmed up?
Remember that I play in a band, not an orchestra. For us, a long tacet section is six bars. I really think the horn's temperature is stable and not related to this issue, but I have given it careful consideration.
Rather than your [paraphrase] "chops falling apart", it could be that you're now "into" that instrument enough to actually have rightfully "found" the "center" - and (possibly) the "center" is low. I've not noticed any particular "consistency"

from one Holton BAT to the next.
That is possible, and I actually sort of hope it to be true. Saturday, though, I was able to nail pitches and still get the sort of centered sound that is as good as I'm able to get on the instrument.
...and Lew also makes a valid observation. Amateurs rarely play "flat". Well over 90% of this group of musicians seem to follow the adage "better sharp than out-of-tune". I could easily believe that your standmate is playing 30 c. sharp "in general", which surely would clash with some of your pitches being 10-20 c. flat.
This is also true, and the main reason I didn't worry about it for a long time until the oboe player started measuring my pitches on her tuner during our warm-up sessions. It's quite true that another player in our section is battling high pitches, despite having the main slide pulled out nearly to falling out. Funny enough, I've been suggesting a much more relaxed, "let the air fall into the horn rather than pushing it" approach. Maybe I'm listening a bit too much to my own advice and we should meetin the middle, heh, heh.
Rick "accustomed to the sharp tendency more from higher instruments" Denney