Jeff,

Mixing channels out of the 990 with some on XLR and some on RCA is not a problem. It's also not a problem on the inputs to the 7500.

I'm running the in line equalizer for my Kef 107s between the 990 and 7500 and, as it only has RCA connectors, the left and right main channels leave the 990 and enter the 7500 through RCA connectors. The center and surround channels need no equalization and run between the two units on balanced cables with XLR connectors. The amp gain is different on the balanced and unbalanced inputs but this is not a problem, as the 990 allows you to adjust relative output levels on a channel by channel basis.

Reviews of the 970/990 suggest that the 990 sounds better for stereo listening. I've not heard both so I have no direct input on that issue.

Unless you've got some other reason to replace your amp, the configuration you propose should work well.