The RBI is just a preamp (like the EQ section on your amp) and a DI box - it has no power amp, so it couldnt drive a speaker cabinet. Think of it as an external EQ pedal/rackmount unit. Your Ashdown has an EQ section, DI and a power amp built in (the 575 watts rating is what your amp's power section can put out).
So, the only advantage you gain from using a Sansamp really with your amp is the tone - they do sound great. You can use the EQ on the Sansamp and the EQ on your head, or leave one alone, or both, entirely depends on how it sounds to you.