There probably is no spec for stray magnetic flux from the power amp or the sensitivity to magnetic flux for the preamp so the units could very well be in spec.
Of coarse the simple solution is to isolate the two units by physical seperation and the aluminum "eddy current plates" I would probably be curious as to why the amplifier is so loud even though it has a toroidal power transformer. I would probably take the top cover off of the amp and sniff with the loop around the transformer to find the hot spot. It could be the wiring or traces from the transformer through the rectifiers to the input filter caps. Those traces and their returns should be as close to each other and parallel to avoid making a radiating loop. If it is wire you may be able to move the wire to reduce the area of any loop formed. If it is circuit card traces it may be more bother than it is worth compared to the eddy current shield and spacing.
PilotSkier wrote:Both the B&K pre-amp and the power amp were found to meet factory specifications. The unit's original design engineer, after the bench check, suggests the hum is either from a "dirty" AC power source or from the cables. What is perplexing is that the hum is present even without any other component (other than the two B&K units) being "plugged" into the AC wall outlets - this includes the turntable, and no other piece of equipment is plugged into the pre-amp (except of course for the cables carrying the signal and the ground wire from the turntable). As I am typing this posting, I can clearly hear the hum from ten feet away from the speakers. Out of frustration, I pulled the pre-amp off of the top of the power amp. As the pre-amp was being moved the hum got louder (the same as it did with the 1" coil probe), then diminished, got louder, but then started to diminish as the pre-amp was removed from the amplifier's vicinity. When the pre-amp was about six inches away from the amplifier, the hum was almost gone. (So much for marketing hype!)