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The tuba ("basses") part I have for 1812 in the band arrangement we are doing, is at least similar to the "Banda" part on imslp. Which I found out because I went looking for the original because the speed of double tonguing required for the big noise part that is the last section of the piece, was pushing it for me. But those notes weren't even in the part labeled tuba in the original, but were in the part labeled "Banda" and were slurred, not tongued. Slurred they are no problem at all, and that is a major boo-boo on the part of the arranger based on the difference in how it would sound.
But I have no idea what a Banda is even if it is showing up on imslp as part of the original set of instruments. Especially since there is also a tuba part.

The banda parts for 1812 are extra brass band parts (with flexible instrumentation) originally written to reinforce the orchestra. I wouldn't consider them as primary source material for a band arrangement, but would instead look at the main orchestra parts.

Can you be more specific about which passage you are referring to? I would guess that if it's not in the original tuba part, it was pulled from a string part instead.

I've always seen it used to denote an offstage part (usually brass).

https://ks15.imslp.org/files/imglnks/us ... ussion.pdf

Number 27.
The tuba part in the original orchestral 1812 doesn't come in until the 9th measure after 27, and it mirrors what the Banda part is doing there.

The other available part for this band version is a BBb treble clef part, which doesn't look like either of the above in that area of the piece.

Having now seen what the original tuba part does, I am inclined to just do that. But it's not what is in the score as the tuba part, and I could get questioned about it. However I suspect it would be cleaner overall.

If the tuba section can cover the full passage starting from figure 27, they probably should. In the orchestra original, the full passage is covered in the strings, with the tuba only coming in at the very end. In a concert band version, you don't have a string section covering all of that, so the tubas have a play quite a bit more to make up for a lack of celli/basses.

Not knowing which concert band arrangement you are doing nor having access to the score or part, it's hard to say if the line is sufficiently covered in the low woodwinds that you could come in later. Might be worth a conversation with the conductor to find a good solution.

Since I have access to all the parts (this is a download-parts band) I will see if anyone else has that octave and make a decision based on that. The conductor has a sub next week, and only one more rehearsal after that. I can do it just fine slurred but if there are obvious others doing it, I may still take the route of not doing it, especially since the part labeled tuba doesn't do it at all. I had "assumed" there would be other tubas, but there are not. That other part is more for a contrabass, and the one i have fits pretty well on the Eb. And doesn't make me read BBb treble clef. (I can, but it's a hassle and would require quasi-memorizing.)