Unless I've missed the point a rheostat IS a variable resistor (just wire wound) What you need is and Lpad which is very different. (2 rheostats with 1 common wiper)
Either way if you wish to attenuate a 100 Watt amp you will need a lot more than a 100 watt rheostat otherwise it will overheat and likely burn out and take the Amplifier with it.
Search for *attenuater* on this forum as there have been a few posting on that subject,, have no time right now to find them.
Phil.