hello,
I believe a slave amp is nothing more than a poweramp?
you basically need a pre-amp section to contour your tones.
There are many ways to use it, but basically you have a head and a cabinet, the head needs a DI or pre-amp out which would be run into the front end of the slave and then into another cabinet/load to handle this power, so you basically end up with two half-stacks that share the same pre-amp.