Thanks, gonk - I never tried passive biamping, just biwiring. It must require Y connectors from the preamp to feed the two amps in each channel. Makes me wonder what my results would be with my nOrh monoblocks connected to the 1.6 woofers and my old Dyna Stereo 70 to the tweeters - assuming it still works, and both amps have about the same gain, and the 990 accepts their combined parallel impedances.