You can certainly split the pre-amp output and connect to unused channels of the amp to bi-amp. The signal will be full range to both amps (although the 990's bass management will subtract some information away for the subwoofer, ranging anywhere from 40Hz to 150Hz or so depending on where you set the crossover for the mains). If you wanted to have a low-pass filter of some sort on the channels driving the woofers, you would need something in addition to the splitter - an option that occurs to me is Outlaw's ICBM. You could connect the left and right pre-amp outputs to the ICBM, set it for stereo subs, and use the left and right subwoofer outputs to feed the woofer amp channels while using the left and right outputs to drive th eother amp channels. The standard ICBM has a max crossover point of 120Hz, which wouldn't quite work if you were looking to have a 180Hz crossover, but the Magnepan edition of the ICBM uses higher crossover points (I believe it ranges from 80Hz to 240Hz in 40Hz increments).