Passive bi-amping will give the speakers more power, so if a single channel of the amp by itself won't do it then two should give you the extra you need. I'd run a "Y" connector with the unbalanced outputs so you maintain the same gain (balanced produces a signal that's 3dB higher).

The 990 does have channel calibration controls that allow +/- 15dB of control over each channel. This is necessary with any surround receiver or processor to allow the different speakers in the system to be adjusted to produce the same volume levels. The 990's manual can walk you through both the automatic and manual calibration processes.