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For many of the jazz band gigs I have been playing in the last 5 years or so, the band leader wants us to play sitting down. I don't have a traditional Sousaphone stand (haven't since Junior Highschool) so I have worked out several comfortable playing positions.
The one that works the best is to turn the bell to the right, rotate the gooseneck to the left and rest the horn on my right knee and front of my right shoulder. It looks a bit strange, but it works and my "good ear" is much closer to the bell - I think I play a bit better than when in the normal left shoulder position.

Nothing earthshattering here, just mentioning what I figured out...

I've just spent the last three days (and nights) listening and watching brass bands in New Orleans at the French Quarter Festival. Most of those guys drape the sousa across their left shoulder with the valveset about centered on their chest and have the bell pointed slightly to the left. None of the bell in front of their face stuff.

Just make sure your chair does not have a large backrest or arms.

Yes, getting a chair without arms or a high back is a good thing! Sometimes it takes a bit of scrounging through the stacks at some places, or bringing your own. My preference is to play standing up with the horn in the "regular" position, but after two long sets sitting down I have been experimenting during the last hour.
I found this position to be quite comfortable and it took the weight of the horn off my left shoulder.
I am sure it is not right for every situation, but it works!
(I usually take a folding stool for long sit-down gigs but I didn't have one low enough to match the rest of the band.)

The leader of the "Sinful Saints," a Dixieland band in which I play,
INSISTS upon a sousaphone, not a concert tuba nor a recording bass, for all engagements, including the church performance pictured here. I surmise that this is as much or more for "impressive" appearance's sake as for sound projection.

However, most of our performances are on top of a fire truck in parades. Since the truck is constantly stopping and going without warning, the raised center of gravity makes standing with the sousaphone out of the question (not to mention the increased risk of the bell tangling with tree branches).

The sitting position that I use is fairly close to the traditional standing position, as pictured here in a church gig with another Dixieland band, the "Mad Monks of Melody."
If I can be comfortable supporting the weight of a borrowed 46K Jumbo sousaphone (thank you again, Andy!), my left shoulder can certainly bear the weight of my 36K fibreglass sousaphone with the 14K brass bell.

The trick is one learned from Alan Baer's suggestions for auditions: always try to eliminate the variable elements. Therefore, I always bring my little metal round folding chair so that I can predict the height and be free of obstructions as much as possible.