There are many, many ways to go on this one, I'll try to outline a few easy ones.
The stock attenuator configuration has those dropping resistors that go from the selector switch out to the terminal strips close by. These are useful to pad the incoming signal to the Foreplay preamp circuit, which allows for the 12 positions on the Sweetest Whispers controls to get you in the sweet spot.
When I have a Foreplay III come in for repair, I will generally remove both controls and all the series resistors. I then put in a second 4P3T rotary switch and a 50K stereo log pot. I send the output of the input selector switch to the second selector switch, where I have a series string of resistors to pad down the input (think of it as coarse attenuation). This would be something like a 50K, 100K, and 250K resistor strung in series on the selector switch, then the output of this control goes to the 50K stereo pot. This gives the convenience of a stereo control with the ability to pad the input so that you can use the volume pot where the channel balance is good.
Another route I have seen taken is to put a pair of 100K mono attenuators in for each 15K attenuator, wiring them as conventional volume controls. You can probably leave the padding resistors alone in this case. What I don't like about this is that the 100K controls have many steps, and it can be tedious to get each one set on the identical position.
A third option is to put a 3/8" metal hole cover in one of the holes for the SW, then put a Stereo 10K log pot in place of the other control. This would actually be very similar in terms of gain to the SW, with the lowest possible cost and least amount of work. You could wire the control in as it is in the stock preamp (which ensures that the grid always has a path to ground), or in a more conventional manner.
If you plan to sell the Foreplay III in order to finance the purchase of the Stereomour, keeping an eye on the resale value will be pretty important.
