For anyone else looking at this, here's the answer:
- The attached circuit had positive feedback - I forgot to flip the op-amp to put the - terminal on the top to match the circuit I was duplicating. The feedback needs to go to the (-) terminal, not the (+).
- I forgot to adjust the feedback resistor resistance. In order to have a direct adder, you have to have R1 = R2 = RF. Since RF is 10k and R1 and R2 were 1K, I was getting -10 x (6 + 9) = -90V, which was exactly what I should have expected.
- Op-amp adders will always invert, that's why it's negative. To flip it positive, would need another stage.
This was less re-learning simulink & simscape and more double-checking my values to make sure they were correct..