TromboneChat

Bass trombone - Tips to improving tuning over octaves? B flat in the stave is much flatter than Bflat above!
I guess the problem is mostly the user although I have heard that deeper mouthpieces also have an impact.
Which Bflat should I use to tune with!!

the 2nd partial (low Bb in the staff) has a huge, huge slot. There's a lot of space where that note can sound "acceptable" but not necessarily be in the center of the note. Experiment with where you play it.

I always tune to the 3rd partial with the ensemble or when I tune while practicing. And also 2nd just so I know where it is also. Tune all your notes, pretty much, as you play your scales make all the effort you can to hit the center of each pitch right away. Whichever octave you tune to make sure you’re moving your slide to play the next octave in tune.

I like a bass to have some tighter slotting so I don’t have to work too hard to center my notes, but not toooooo slotted so I have wiggle room for other reasons.

If octaves don't line up it COULD be the mouthpiece backbore being wrong for the horn, or the valves slightly out of alignment. It could definitely be just you though, too.

use the one that's furthest "in" to tune, then keep working on developing your embouchure (making sure to buzz in the middle of the pitch at maximum resonance) and sense of pitch in the long term

Is the top Bb even sharper and the pedal Bb even flatter? If yes, it's very possible it's the mouthpiece having too large a throat/backbore for the horn (if the cup was too deep the problem would be reversed, higher octaves would be flat).

If not, if only that partial is out of line, then it's likely you, or a quirk of the horn.