This is richly debated elsewhere, and it occurred to me that some here may know more than the average tuba player.
In an 1871 letter preparing for a performance of Aida at La Scala, Verdi wrote "I wish to insist once again on a fourth trombone. That bombardon is not possible……I would prefer a trombone-basso which is the same family as the others; but if this turns out to be too much trouble or too difficult to play, then get one of those ordinary ophicleides that go down to low B."
So what was "that bombardon" he was anxious to avoid turning up in his opera pit probably like? And what was the "cimbasso" for which he scored some operas like? (NB: it was not the modern cimbasso, which is a creature of the 20th century.)
Any lore?
Tubist here, I'll try to tackle this question as best I can.
From Roland Jackson, "Performance Practice: A Dictionary-Guide For Musicians":
Bombardon (19th c.): A valved tuba of wide bore used as the lowest brass instrument in Italian opera orchestras. Although Verdi, from Oberto to Aida, designated this part a cimbasso, he may actually have had in mind a bombardon, since it (or the ophicleide) had generally replaced the cimbasso by the 1830s. For Otello Verdi expressed a preference for the contrabass trombone over the bombardon (or cimbasso) in that he felt that it blended better with the other brass instruments.
By the time Verdi premiered Aida, the cimbasso had come to mean virtually any instrument below the trombones, be it ophecleide, bass trombone, an actual cimbasso, or the bombardon. The Oxford Dictionary of Music tells me that the the bombardon could have either been referring to an early version of the tuba or the bass shawm. Personally, I'm inclined to think it was the bass shawm that Verdi despised so much - I've listened to it before, it almost physically hurts.
However, Verdi did consult the Pelitti workshops in Milan to create a new instrument with the precise sounds that he wanted. This instrument, known coloquially as the trombone-basso Verdi (or the Verdi bass 'bone), was in use until the 1920s in Italian orchestras and pits.
Sources consulted and further reading:
http://udini.proquest.com/view/the-cimbasso-and-tuba-in-the-goid:860121953/
http://digital.library.unt.edu/ark:/67531/metadc31531/m1/8/
http://www.tubaforum.it/Lists/Articoli/images/james_gourlay_cimbasso_paper.pdf
As an aside, I can personally understand why Verdi would want a bass bone instead in the interest of tone. Blending with a trombone section can be tough at times, especially during faster passages (as in Aida). Trombones have a "borier" and brighter tone, and the tuba part in Aida can get relatively high and technical in certain spots, so it's understandable why Verdi would want a bass bone.