Unless I'm entirely missing something, there are two capacitors which go to ground after each resistor, ignoring the muting transistors (FETs). This would create a 2 pole filter if this is what's going on. They could be for click suppression in muting too, but unless we tear the unit apart, we'll never know. They could also be resistors, but why would they use this style SMD resistor when all the others are a different style? In any event, if they are using these series resistors to avoid loading the opamp outputs, they're choosing higher values than they need to do. There is a 'pole' created by the capacitor in the summing stage, but not knowing its value, its impossible to determine where in the frequency band this is effective.