Passive Biamping brings questionable performance additions. You could always try it and trust your ears. Provided your Speakers are 8ohm Nominal, have you considered using the BTL Bridged Capability instead?
As opposed to Biamping, it will provide you with a good amount more power to your Mains.
You will probably increase passive power draw and heat wasted at the crossover circuits as well. If you are active bi-amping that's another concern entirely but then again you may end up with worse performance.