If you mean active biamping I think you'd feed the two front signals to a separate line-level crossover network and feed the four outputs from that to four amplifier channels, on a seven-channel amplifier, two (preferably identical) stereo amplifiers, four monoblocks, or whatever, each of which would connect to one of the tweeters and woofers of your main speakers, bypassing the speakers' internal crossovers. The center and left and right rear outputs of the 990 would of course each feed another amp channel which would drive the corresponding speaker. The 990's left and right surround channels wouldn't be used. This is more straightforward than it sounds. I'd be interested in the aural benefits.