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I'm struggling with my mid range. I am very strong in the low registers, pretty solid down to pedal G, 7th ledger line below the staff. Anything above Bb in the staff I struggle to center the notes. The D is ok open but to play it third valve or 12 combination, I can't center the pitch. Above that, I'm mediocre at best.
I'm using a King 7 mouthpiece with a 33.25mm cup diameter. If I use a smaller mouthpiece like the Helleberg, my low range is sacrificed. Ive tried the Schilke 69C4 but its too small. The smaller pieces improve my mid and upper range but then loose response down low. I like the old King MP, I have tried a variety of others but the King is comfy.
Is this a mouthpiece issue? How can I strengthen my mid range? I use the lower range much more than anything else. Your thoughts or advice? Any exercises I can do to work on this?

I'd be a little concerned that there's something "wrong", mechanically, with your technique, and it might not help at all to just hammer away at it, or switch mouthpieces or whatever. You could have been describing my range, for more than a few years when I took the tuba up as my first brass instrument. It was frustrating - like you, I see the tuba as mainly about low notes in any case, but it's awkward to be having trouble with ordinary band parts. It isn't such a problem now, but I can't really say what happened and I'm certainly not in a position to advise anyone else on such matters. But if I could have found someone whose diagnostic/instructive abilities went beyond better breath control, it might have shaved a decade off my learning curve.

What are the dimensions of the Helleberg 120 and HII? I had borrowed a Schilke 67 and it seemed too small.

Obviously it is impossible to know what is going on over the internet, but your description sounds allot like a tuba player used as an example in the embouchure types video serious on youtube by wilktone. It is a six part serious. Anyway the tuba player in had a problem with splitting notes and double buzzing in the mid range. The problem was he was between embouchure types. He used a downward blowing embouchure for the high range and shifted to a upward blowing for the low, the fix is keeping the embouchure type consistent for your entire range. I believe he addresses the tuba player I'm referring to in the 5th or 6th video. Anyway it is a possibility.

There are many possibilities and it's not really possible to diagnose without seeing it. The issue of being midway between upstream and downstream is one possibility. One lip needs to predominate, and when there's a conflict anywhere, that range it can be very unpredictable.

From the decription, it could just be that you haven't found a mouthpiece that works for you. "Comfy" doesn't necessarily mean it works well for you. A wide rim may be locking you into a mouthpiece placement that is not ideal. Try something with a narrower style rim that is still comfortable, maybe a Schilke Helleberg II.

I just saw this... It's not the way I would say it, but there are some valuable ideas here, that apply to the question:
http://www.jayfriedman.net/articles/ear ... s_the_note