Yeah, that really isn’t fun for stamina. You need a lot more strength in your jaw, when you don’t have a mouthpiece to hold you in place. And especially when flapping your lips at the frequencies needed to produce tuba-level tones, it is quite likely for your lips to break open.

Basically, I don’t even want to hold a tone for 10 seconds, because I can feel my lips getting battered. Presumably, this could be improved, though, by specifically training without a mouthpiece.