Hear a Cimbasso and Other Low Brass

As a brass contrabass, orchestras generally use a tuba. It's a long-wide-flared saxhorn with soft deep sound. In wind and brass bands, it blends in the saxhorn group, but not with the short-narrow-flared trumpets and trombones with brilliant sound. In symphonic orchestras, until they have a saxhorn group, the tuba is precious but it blends with nothing.

Wagnertuben, already mentioned elsewhere, will be the contrabass and bass of the horn group when more composers eventually use them.

At Berlioz' time existed low trombones with a longer slide and a handle to prolong the musician's arm, or with two slides side-by-side used simultaneously. Record:
xM5s0l3sUwo 5:34-5:40
Nearly all present bass and contrabass trombones have the tenor's slide length, with wider bore to favour low notes, and they may have more valves to reach seamlessly the pedal notes and extend the low range. Records:
vsl.co.at bass, vsl.co.at contrabass

Precisely because the tuba doesn't blend with the trombones, Verdi and Puccini demanded a Cimbasso, which is presently a short-narrow-flared contrabass with valves rather than a slide. The nice sound differs a bit from a low trombone, the technical possibilities too. Records:
vsl.co.at
C2wvykZOwwM compared with a tuba, 0:42-0:58, 1:56-2:19, 3:42-4:43
the instrument is rather common in Italian orchestras but unusual elsewhere.