(I realize this is a rather old question but there are other options.)
To be pedantic, it is not necessary to have Amanda even installed to read backups off those tapes. Having Amanda just makes it a lot easier. I just recently experienced this sort of failure and didn't find the information I needed from the current Zmanda wiki, but I did find it in their deprecated section with Google. (See below.)
The front of each tape will have a 32k header block containing plain text, and that text contains instructions how you can recover the backup files off that tape using normal *nix commands. So, to read that header, all you need to do is:
dd if=/your/tape/device bs=32k count=1
and that will dump the text header to stdout. To read the tape's contents
If you're using physical tapes, each 'record' on the tape contains the backup of a single filesystem, or DLE, and each one will have a 32K header on it with instructions and metadata specific to that client. The content of each block after the first is whatever your Amanda was configured to create, which can be gnutar, star, cpio. or something custom, and all can be compressed or not compressed. As these can vary by client, each record on the tape can be a different format, but all of them will have that 32K header block with instructions.
If your tapes are virtual tapes using the dsk-changer driver, then each DLE is written to its own file, and finding what you want is a lot easier because the host and disk are part of the filename.
This old web page at http://wiki.zmanda.com/index.php/Restoring_files may be deprecated, but it still was very useful to me just two weeks ago, when there were "interesting times" in our data center and my Amanda server's VM rolled back about six weeks (because the VM is hosted in windows and windows crashed, taking my Amanda VM with it.) I had the Amanda binaries, but the tape databases were all older than my existing tape library, so I couldn't use Amanda's smarts to read anything off my virtual tapes. Yeah, today I'm building a new bare-metal box to be our Amanda server, so windows won't destroy me again.