Mark Rubin by David Kaufman
The tuba holds down the bass line, but some players are more active than others and you can sometimes hear them doubling bits of melody and including tenor lines.
Klezmer tuba players in the late 19th and earlier 20th centuries included:
USA Izzie Drutin
Klezmer tuba players in the later 20th century and today include:
Russia Alexei Stepanov, Paul Tkachenko (also double bass)
UK Oliver Presman
USA Mark Rubin (also double bass)