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Well, you're probably playing Baroque pieces for this gig, so play like the bass pedals of the organ, strong and present. Chances are, you'll be the only Tuba, so having a horn to present the bass line with a full sound will be needed, and that 186 you have is a horn that can fulfill the job, since its a small ensemble probably.

And no matter how loud you play, the trumpet players will always find a way to play louder, so don't over do it. Just make it fit with the group. You don't have the melody, so you don't need to play the loudest, you need to play the most full, menaing you fill the room with the bass.

I'll be the first to say it:

LISTEN!!!!!!!!!!!!!!

There, I said it....
Listen to your fellow brassmen, and know what kind of purpose you are serving in the music. 99.99% of chamber music is knowing where you fit into the other parts.

If you're getting together for a rehearsal or two, I'd recommend recording the ensemble for all to listen to. One hundred percent guarantee that someone will be too loud,