Rick, 100k will likely be too high if you want the pot outside the box and will be running cabling from the box to the amps. 100k is fine if you mount mono attenuators inside the amps. With the pot outside the box and connected by cabling to the amps, I wouldn't go over 50k, and more like 25 as others have said. Generally you want the source to have as low an output impedance as practical (in this case it is fixed and a bit higher than typical), and a high impedance on the amp end. When you put a high value pot somewhere in between, you are in essence, raising the output impedance of the source by quite a bit and the capacitance of the connecting cables will roll off the high end and generally make the sound less dynamic, etc.
HTH,
Jim
Jim Rebman -- recovering audiophile
Equitech balanced power; uRendu, USB processor -> Musette DAC -> 5670 tube buffer -> Finale Audio F138 FFX -> Cain and Cain Abbys near-field).
s.e.x. 2.1 under construction. Want list: Stereomour II
All ICs homemade (speaker and power next)