earliest 'serious' tuba solo repertoire

[Todd S. Malicoate]

Here is a go at defining:-

Serious music generally requires more than one listen to fully understand and reveals more with repeated listens.

That is against light music which is written largely to entertain and be immediately appealing, but can get boring with repeated listen.

However what music comes into which category is blurred. There is much light music which is also quality music and repays repeated listen.

Your definitions presume that all listeners have the same level of understanding and the same likes/dislikes. Clearly that isn't the case.

Some highly trained musicians will "get" the intricacies of Mozart on the first listen. Others will never recognize the detailed counterpoint.

Some can listen to rap or hip-hop over and over without getting "bored." Others will reject it utterly.

[ScottM]

The Chaconne by Leo Sower by was published in 1938. You don't hear it but it is rather pretty piece. It is about a grade III.
ScottM

[Todd S. Malicoate]

Some people get their turtlenecks too tight. Good music is good music, be it "art" songs, concertos, or singing through your tuba to a potted plant. All serious music is not good and not all good music is serious.

I don't disagree with you at all, but I still have no idea what the OP means when he says "serious" music.

It's kind of important to understand that in order to answer his question.

[Mark]

The Storm King is one I'm not familiar with. It seems, though, from the pieces you guys have come up with, the Tcherepnin and Gardonyi may in fact be the first serious, non-program musical pieces (art music?) for solo tuba.

A serious grad student once told me that Berlioz' Symphonie Fantastique was program music and therefore could not be serious music. Seriously.