you using the stock mouthpiece or a replacement?

Some European instruments have a different mouthpiece receiver that uses a different mouthpiece shank taper than what we usually use.

Check to see if the mouthpiece that came with Besson tuba goes in further on the shank than your replacement mouthpiece. If your replacement does not go in as far that is why the tuba is playing flat. You have made the tuba longer with a mouthpiece that does not fit properly in the receiver.