An unusual trick that can work wonders is to flip the AC supply polarity of the offending device, in this case the preamplifier.You can only do this if the preamp employs a 2-prong AC connection!
Ground loops occur when different parts of the system share a ground in such a way that noise in the ground is imposed into the signal at an earlier part of the circuit. The best way to eliminate it is to break the common ground- usually at the amplifier end as this is where most of the current usually is. If you can float the amplifier's AC ground at the power cord (using a ground cheater available from a hardware store) and that eliminates the buzz, then there are steps you can take to shut things up.