So, move really close to the bridge, more weight (but not more pressure, they're different), SLOWER bow to avoid the scratchy sul ponticello sound. It will take quite a strong pull to keep the bow moving like this, because you have optimised the coupling between bow and string. Practice so it starts cleanly. Try this in a practice room and it will sound uncomfortably bright and fairly harsh as well as really loud... add that to the brass and it'll be awesome.
As has been said, you're not competing with them, the Tuba is a bass instrument that comes in BELOW you in the blend, so you're now playing accompaniment to them; you don't want to be going for boom, you're now aiming for a sound that will change their character and add some stringy cut to the brass mellow.