Hello! Just wanted to pass along some names for you to listen to that you may or may not be familiar with, as well as some advice. I'm sure the older and wiser guys can scrounge up many more than I can, but I've got a few that I listen to:
There's Alan Baer with the New York Phil, Øystein Baadsvik, Arnold Jacobs formerly with the CSO(of course!), Bill Bell formerly with the New York Phil and David Zerkel at the University of Georgia (He has a couple of CD's floating around). Those are the five that I listen to the most, but there are countless more. Just find someone who has a beautiful sound, and try to emulate them.
Tonguing is something I think most young tuba players struggle with, myself very much included. Try to get your hands on some tonguing exercises, which can be found with a simple Google search. Work these up starting with a slow tempo and GRADUALLY increasing, and over time you'll see great improvement. One of the most important things to remember is that no matter how hard you try, you will not turn into a fantastic tuba player overnight. It takes time. Lots of time and lots of practice. Also, practice consistently. It doesn't necessarily have to be an Enormous amount of time, as long as it is consistent. Make sure to practice your weakest areas of technique, don't just repeat exercises which show off your best areas. A great tone builder is just a long tone exercise, holding each note until you find it to be of satisfactory sound. Make sure to focus on using your best sound, achieved by making an O shape with your throat and blowing fully. Another important point is to make sure you realize that blowing with a fuller sound is NOT the same as playing louder.

A great point that David Zerkel made at a clinic this January was that when you blow, don't just blow to the end of your mouthpiece, blow to the end of your bell.
Just some random practice advice I pulled out of my brain. One more tip, amass as much literature for the tuba as you can. It comes in extremely handy.
Happy tubaing!!

Feel free to PM me if you need anything.
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