Stuffy Low Register

[TinyTubist97]

I have a Getzen G-50 and I have trouble getting the proper sound on songs like Prokofiev's Romeo and Juliet, Fountains of Rome, and most other low and powerful songs. The tuba really sings in songs like the Vaughan Williams though. Is there any way I can combat this issue? I'm a 5'1'' freshman in high school so maybe It's just because of my size.

[TinyTubist97]

And I use a PT-50 mouthpiece. I know it's huge but I've tested many other mouthpieces and it just seems to work perfectly for me.

[bort]

Don't blow too hard in the low register. Those horns take different air down low than in the mid range.

Practice by playing softly and hitting all the notes. Once you learn where they are, then you can start thinking about playing louder, a little bit at a time.

Take stuff you know how to play and practice it an octave lower. It will be hard, sound like crap, and not be fun at first. But stuck with it and it will get a lot easier.

[toobagrowl]

I have a Getzen G-50 and I have trouble getting the proper sound on songs like Prokofiev's Romeo and Juliet, Fountains of Rome, and most other low and powerful songs. The tuba really sings in songs like the Vaughan Williams though. Is there any way I can combat this issue? I'm a 5'1'' freshman in high school so maybe It's just because of my size.

Even pros have to work at getting a big solid sound in that register on those pieces, I think. You are small and still very young; it will come with time and lots of practice.
Use lots of air (deep, quick breaths) and a more open oral cavity (not TOO open, though) with a good buzz on those lower notes.

I think that the PT-50 may be a bit TOO big for you. Mouthpieces that are either too small or too big wont work as efficiently as mouthpieces that are "just right" for your embouchure. A very large mpc may respond slower and come across as "stuffy" if it is too large for your embouchure and/or lung capacity.
Heck, I am 31 and over 6'1" tall and have decent lung capacity and can fill up my large bore tubas pretty well. But I find mpcs like the Bach/Benge 7, old PT-0 and even the PT-88 or PT-50 to be a bit big for me because my mouth/embouchure is smallish. I have used some of those mpcs and can "get by" with them, but also have more "air balls" with them (which irritates me even more than chipped notes )
I am most efficient on 'medium bowl' mpcs like the Faxx/Bach 18.

[pgym]

And I use a PT-50 mouthpiece. I know it's huge but I've tested many other mouthpieces and it just seems to work perfectly for me.

Are you judging from BEHIND the horn or from IN FRONT OF the horn (the audience's perspective).

This observation from Doug Elliott may be of relevance:

If a mouthpiece (rim size in particular) is too big for YOU, it will sound big from your perspective but will not get a focused sound that projects. Moving down into the size range that DOES work for you will make your sound more focused and sound better (and bigger) from a distance.