Markbass even states that you can in their FAQ:
http://www.markbass.it/support.php
"Can I use my Markbass amp without connecting to a speaker cabinet?
Yes, any Markbass amp can be used as a preamp only. We recommend that when doing so, you turn off the master volume (not essential, but it's better for the amp). The gain control and/or the Line Out level control will regulate the output from the amp."