I obtained the following from the command 'amtapetype -f /dev/nst0'

We have an ATL ValueLoader DLT VS640 which is a tapeloader - the underlying tape drive is the VS80.

Our backups are working correctly, using these values.

Writing 256 Mbyte   compresseable data:  38 sec
Writing 256 Mbyte uncompresseable data:  105 sec
WARNING: Tape drive has hardware compression enabled
Estimated time to write 2 * 1024 Mbyte: 840 sec = 0 h 14 min
wrote 1107222 32Kb blocks in 3386 files in 23483 seconds (short write)
wrote 1106281 32Kb blocks in 6787 files in 33118 seconds (short write)
define tapetype unknown-tapetype {
    comment "ATL ValueLoader DLT VS80 VS640"
    length 34625 mbytes
    filemark 8 kbytes
    speed 1288 kps
}

a note:

the comment originally read: "just produced by tapetype prog (hardware compression on)"

As for this drive: http://amanda.sourceforge.net/cgi-bin/fom?file=414 
there doesn't appear to be a way to turn off hardware compression