Why does an diff-amp being fed by two separate op-amps with low input resistances distort the signal on the inverting input?
Realize that the input impedances are quite different on the two inputs. On the non-inverting input it will be 120 ohms to ground, and on the inverting input only 1/6 of that.

The outputs of your max9643 chips can only supply a few mA so the inverting input will load down the source excessively with &lt;100mV input.