Could you convince your DM to let you flavor your shield as a tuba? The idea is definitely within the incredibly silly category but if that's your DM's cup of tea it wouldn't be game breaking for it to just be a shield in all but name; remember that most Bards are using Oratory/Sing/Dance and not carrying an instrument at all. You're not getting anything more than them with your more novel concept.
Why not take Perform (percussion instruments) and wear a breastplate made from a washboard or a xylophone, a shield made from a cymbal or a gong, a helmet made from a bell and wield a cabasa or a shekere as a mace? That's an entire suit of armor and a weapon. Better yet, your armor would be made of stuff intended to be struck so you'd get to make Perform checks or use Bardic performances when people hit you.
|Darigaaz the Igniter|
The tuba was only invented in the real world less than 200 years ago, so I guess you'd need to clear with your GM it's permitted in his campaign anyways.
But if you get it and are using it as a shield, I'd expect to find it targetted with sunder attempts, leaving you without an instrument. Also a tuba weighs about 25 lbs, a strengthened one as a shield would add somewhat to that. A regular metal heavy shield in pathfinder only weighs 15 lb, so your tuba-shield would be unwieldy and have a huge armour check penalty.