gonk, now that you mentioned the balanced outputs, let's open that can of worms.
990 has balanced outputs, which means one phase splitter per channel just before the XLR outputs(8 total). Also, the receiving power amp needs one differential amplifier per channel to implement the common rejection mode (7 total).
Thus, per channel, you have two active components in the analog signal path only to reject the noise picked up by a pretty short interconnect in most cases, running from the processor to the power amp.
I guess you see the question coming...
Do you think this is worth it? Do you really feel a positive difference between connecting the 990 to its power amp in balanced vs non balanced mode?
I "feel", for I haven't tried it myself, that the addition of the phase splitter and dif amp in the analog signal path adds up to be more distorting than the minute noise picked up by the high quality interconnects.