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That design is nice. Simple and elegant. I would run it at 18V rails, or even 20V, to increase headroom. In this application, where people are going to try and run their individual protools channels as hot as possible, headroom in the summing amps will be important. Increasing the supply voltage doesn't cost anything extra.
The other thing I would change would be to improve the simplicity by eliminating the output capacitors from the amplifiers. DC coupling the amplifiers to the transformer would be safe if you use good design practice to minimize and equalize the DC output of the two amplifiers. If they're equal, then no DC current will flow in the transformer and it'll be happy. To be safe, you can minimize the output offset by equalizing the input offsets on the inverting and non-inverting inputs of each amplifier. You do that by giving the non-inverting input a resistance to ground that matches the net effective resistance-to-ground of the inverting input. In the case of a summer with 8 inputs, this would be 2523 ohms. The catch is that the ideal non-inverting input resistance will change, depending on how many channels you had in use. There are ways around that.
Of course, you don't need the transformer. The output is already balanced without it. Shave a couple hundred bucks off the price tag. I would still DC-couple the outputs.
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