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How much does bore size matter? I have a loose understanding of it; from what i've read, I gather that larger bore instruments generally sound darker and require more air. There is something in particular i'd like to ask about: If I were to get accustomed to playing say, for example, a king 2341, with a .687" bore, would it be considerably difficult to switch to something with a bore the size of .750"-.800" ?

There's a lot more to the horn than just the bore size - in my experience (which is strictly amateur, but a lot of it) the large bore horns don't necessarily need more air to produce the same volume, but will accept more air, allowing more dynamic potential before the tone breaks up. This is a gross generalization, since every bore/valve/leadpipe/mouthpiece/bell and player combination is different, but if you have the lungs and chops to keep it together, the larger bore horn will be capable of a bigger sound. As an example, my Rudy, with a .866 bore can produce a pp tone with less air and effort than I could on my little MW182 with it's tiny .689 bore, but will bury the 182 in the ff's - but accept three times as much air doing it. (Of course the whole horn is about 3 times as big too, so who's to say it's only the bore size?)
As far as having problems with the larger bore horn, all you can do is find one and try it - but don't give up easily - you will grow into it if you give it the effort it needs. The horn is a resonator - not an organ pipe, and more air won't necessarily give you anything back unless it gives you a better buzz .

Bore size can affect overall tonality, but only in a marginal sense.

The assertion that a larger bore "can accept more air" is bullshit. After the velocity of the breath passes over the embouchure making it vibrate as an application of Bernoulli's principal, then after transitioning the narrow part of the throat of the mouthpiece, the backbore and leadpipe expand so that air velocity drops to approching zero, just like the velocity of a river drops to near zero when it reaches the delta transition to a larger body of water. At that point, static wave theory with the compressions and rarefactions take over and the resulting resonances are what we call pitch and tonality.

Why it will feel like it needs "more air" or "feels stuffy," or alternatively, "blows freely," is a function of something else, on the downside, like a misplaced brace or bend in the tubing, (or a sock, or....) that impedes a node or antinode, so that mechanical impedance rises at that point, and on the upside, using the Rudy as an example, computer aid to place the various parts of the horn to make it as efficient as possible. Likewise to the above, a 38K with its .734 bore, because of its efficiency, can play a whisper or rattle the mortar between the bricks, including wonderful "false pedal tones," while the 3-valve comp Besson I used to own had about the same bore and length of leadpipe, but with a smaller diameter bell of a different taper, and the greater number of places that nodes and antinodes could be interrupted due to the comp valve block, had a really stuffy C & Bnat, an open low F and E nat, but no usable "false" pedal tones.

I played small-bore tubas for many years because that's what was available, but was always frustrated by the way the tuba would fight back when I wanted to play a loud low note that required a lot of valve tubing. I don't mind some "resistance" in a tuba, but I don't want it to change drastically when I depress pistons.
Now that I have some larger-bore instruments I am much happier. In my opinion the resistance should be mostly in the mouthpiece and leadpipe, and not so much in the valve tubing.

thanks for all the great answers! I appreciate it