Start with 100 Ohm. If you can get hold of variable resistance, than all you have to do is to move potentiometer till you find sweet spot, than measure your dialed resistance and affix the resistor of identified value.
Usually all you have to do is find load resistor for each channel and simply add(solder) ~100Ohm in parallel with existing 47kOhm one. If you'll have to experiment further than the relationship of total resistance in-parallel is derived with following relationship: 1/rt = 1/r1+1/r2... Therefore if r1 is very large compared to r2 than the total resistance is mostly defined by r2, the smallest resistor as current flows larger on smaller resistance.
The resulting resistance will be slightly smaller than 100Ohm, but would suffice for experiment.