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I'm hoping this is a diagnosable and fixable thing. I have a Conn 11J, that was beat to ****. I took it apart and de-dented the thing and put it back together. Before I did so, I did play it a few times with some groups and found it to be very out of tune. I figured it was partly because I hadn't sat down with a tuner and really put a effort to get all the slides set correctly and so on. Well after sitting down with the tuner post de-denting I found that the open bugle is 25 cents flat on low BBb ( 2 lines below the staff ) with the main tuning slide all the way in. This is with me letting the note find its center ( slot if you will ) before looking at the tuner to see where it is sitting. The F below the staff is pretty darn close to in tune as is the Bb in the staff with the tuning slide all the way in. Pretty much any note below C below the staff is hideously flat. There are a few valve combos that at iffy but managable.

I can play prefectly well in tune with the community bands 186's or 25's, so I am confident the issue isn't in my lack of ability.

The only thing I have noticed that seems different/off is my helleberg mouthpiece doesn't insert into the mouthpipe as far as it does on the 186's or 25's. There is a 1/3" less insertion ( ha I used the word insertion ) into the mouthpipe on the 11J.

After de-denting, was the horn gone over to make sure there were no leaky solder joints and that the valves otherwise are in good shape?

the valves have decent compression, but I haven't looked for leaky solder joints. I know one of the bottom bow joints was leaky and I resolder that joint, but I haven't leak checked all the joints, that I had taken apart. Thanks for the advice, I'll do that ASAP.

If there are solder joints that are sloppy, that could certainly cause funky waves too.
Excessive solder or other junk inside, as well as poorly fitted joints, can knock the waves out of 'true'.

And, 1/3" on the mouthpiece doesn't sound like much, but changing the receiver or the shank will make a huge difference too, possibly more than anything else.

I just did the fill the tuba with water test and the smaller side of the bottom bow had 2 massive leaks. I'm going to take it off and clean that area thoroughly and make sure of its roundness before reassembling it. I don't think I did a good enough job of that last time. We shall see if that fixes the intonation.

These leaks may be stopped temporarily by means of tape. Does that influence the tuning?

Klaus