I think the 770 and 755 are both due to have a new design very soon just like the M200 and the 7100 did. I thought I read that the new 770 and 755 were to get balanced inputs as well. Could be wrong about that. Of course the new amp is going to cost you a bit more than what you are going to be get the older 755 for now.

One nice point for me with the 990 balanced output has been that of the subwoofer output. I use prosound equipment to power my sub and the balanced output allowed me to remove the level conversion device from my signal path.