Bi-amping is not so simple and should not be an option for a while/ever. A nice sub with a built in crossover will let your amp work a lot less and improve the bass. You need to either have pre-amp outs on the roksan or use a sub with a nice built in crossover that will take speaker level in, split the signal to the subs and speakers and have volume and phase controls to allow for good blending.
Look at Velodyne or REL to give you a good idea of what you need.
Look at Velodyne or REL to give you a good idea of what you need.