Wagnertuben (lower horns essentially) were ordered by ol' Richard for his operas. Bruckner and a handful more composers used them too. They would deserve wider use: nice sound, range not covered by similar instruments, and (I believe) available in many symphonic orchestras.

Anyway, here are opportunities to hear them clearly:
https://www.youtube.com/watch?v=mmLRtqGOAJk only music
https://www.youtube.com/watch?v=Hifo18bVG80 some waffle (in German)
https://www.youtube.com/watch?v=j-4xZD6BX_w begins 4:50, with some waffle (in German)
https://www.youtube.com/watch?v=MFm2C-ve7qw 0:00 to 1:40 with waffle (in German) and from 6:23

Interesting. I've recently started revisiting Classical music. I'm not a fan of Wagner, but I recall fondness for Bruckner symphonies. I'll have look at his works.

Very nice records of wagnertuben, chosen to highlight the instrument
https://www.vsl.co.at/en/Brass/Wagner_tuba

The site also tells ranges, notation and more. Not only for this instrument, but also the bass trombone, contrabassoon, celesta, marimba and pretty much all usual instruments of a symphonic orchestra. Definitely worth bookmarking!

An other record: the prelude of Andrew Downes' "Five dramatic pieces for eight Wagner tubas"
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most symphonic orchestras own a set of Wagnertuben to play Mahler and Wagner, so composers could use them more often. More varied than a set of horns, wider range.

Longer record, only music, four Wagnertuben and four horns, again Bruckner 7's adagio
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nice to hear the difference of sound.

More nice music on Wagnertuben, with interesting comments in English
SlU7PMlSOkU - BNcajRV4sfc