Look at dumptypes section in your amanda.conf

By default, amanda uses dump

This one use dump :
define dumptype comp-user {
   comment "Non-root partitions on reasonably fast machines"
   options compress-fast
   priority medium
}

And this one use tar :
define dumptype comp-user-tar {
   program "GNUTAR"
   comment "partitions dumped with tar"
   options compress-fast, index, exclude-list "/etc/amanda/exclude.gtar"
   priority medium
}

Axel Haenssen wrote:

This might sound silly, but how do I tell Amanda to use dump or tar??

cheers
Axel