I don't think it would be practical to install a motor for the volume control for a number of reasons. Probably most importantly, you have to be very careful how you would hook up the motor power supply as it can cause major noise problems if not done just right.
More sensible would be to get a Creek OBH-10 passive preamp (used around $200??) Put your pre outs into the Creek ins and the Creek outs into the amp. Turn up the volume control on your preamp all the way (0 dB attenuation) and you can now use the Creek as a remote controllable volume control. It might add an extra path for the signal, but the Creek is very well done, and it would be much better than anything a pro with a soldering iron could do.