How are others backing up their diskxtender servers that have 25+ million files on them?

I have several diskxtender servers that have over 25+ million files on them.  It is impossible to back them up within a 24 hour period.  So we have just reduced the number of days we back these servers up from 7 days a week to about 2 days a week now.  Is anyone else doing something different that we have not thought of possibly?  Tried doing it as image based backup, but had too many issues to list there.