If I understand the information available at the Polk website, and if you are bi-amping in the normal fashion (for each RTi12, one 770 output feeding the bass section, another 770 output feeding the mids and highs), while you would have ‘400 watts’ total available, the most either section would receive is ‘200 watts.’ (I assume that you are not joining or bridging the outputs of the 770 and feeding each Rti12 with an electrically combined single signal.)

I would think the difference one hears with proper bi-amping is better control of the drivers by the amp and less passive crossover interaction.