You didn't specify whether these were 3592 generation 1 or 2 drives.
If gen 1, the 3592C you have set is the correct devclass specification.

I would check that your tape device driver is the correct type, and is
a recent vintage, to avoid any vagaries in passing device commands
from TSM to the drives.  Defects in drivers are not unknown.  Likewise
for tape drive microcode/firmware.

See if you can find a utility to inspect the drive settings, to verify
what they are.  In AIX, the lsattr command can be used to verify if
compression is in effect; I don't know what might be used in Windows.

   Richard Sims