I have a curved soprano that is good but the bottom C is very flat B and Bb too though somehow I feel C is flattest. It is like the bell joint is too long and those last two tone holes are a little further away than they should be. I took it to an excellent technician and he couldn’t correct it. He described perfectly what I was doing to bring it in tune (“honking in a subtone”). I asked about shortening the bell joint from the joint end to bring the last tone holes in closer. He was resistant to that idea, no doubt sensibly so. The rest of the notes are pretty good and relatively easy to play in tune. It is a Bauhaus curved. Any “sharp” ideas to improve this issue gratefully received as it is portable, a great fun piece of kit and paradoxically has good tuning higher up. Thanks Ed
Don't change the bell joint / length of the horn.
When you "tune" the instrument, center it around low D or E, that might make the higher notes go a bit sharp but also alleviate some of the flatness. In addition, unless you can check it yourself, have your tech look at the key height. If your key opening is too large, those notes will play sharper than they should. Lowering the keys, specifically the lower stack will make those notes play more flat, which will bring the C and lower back into relative tuning.
Also make sure that the C-key opens all the way, you may have to shorten the bumper a bit to give it more room (but I don't know the Bauhaus sop so I can only guess).
Those are the first easy things to check. The next is your MPC because as
@lydian suggested, it could be the MPC/embouchure/blowing angle that causes the issue. What mouthpiece are you using? Also make sure you take in enough of the MPC and blow parallel to the reed without squeezing it. Relax your embouchure to be a non-embouchure and raise/lower the horn's angle - you will immediately know when you hit the right angle/position.