Yeah...I am gonna be the third guy to go this direction. I mean, if someone isn't happy with an aspect of their sound and they wanna try something different...getting a different m'piece is completely reasonable. telling 'em to work on embouchure and longtones is great...it's always great advice....but that will not necessarily get the tone one is seeking.
In this instance....there's no better thing to do than to just take the horn to a shop and try a half-dozen or so. I agree w/ above..maybe focus on your sound concept a bit more so you know what you wanna be hearing.
I also wouldn't say "go for the same mouthpiece Mr. XX played because then you'll be leaning towards his (her) sound"..... because there have been enuff threads wasted on the silliness of the subject of making one's setup like their favorite player's...only to have the person ultimately end up sounding like....themselves !!!!
IMHO (and this may ruffle some feathers, but it is not meant as a negative crit) Cannonball's sound was a bit abrasive...very, very aggressive, very reedy and punchy. So, if that is what you like, you wanna try to find as mouthpiece which will make your Yani sound like that. Because quite arguably, the 991's natural tone leans very closely towards an older-style, rounder/wider paradigm in tone....and if what you want is C'Ball, the horn needs to be moved in a different direction.
I can certainly see why a Meyer 5/6 would NOT be pushing your tone in THAT direction....regardless of how masterful one becomes with long tones and embouchure variation. I think of these sorta Meyers as being m'pieces one would choose to round out a tone, as opposed to punching/edging a tone....
Go to a shop and blow.....Have fun.....