Gentlemen, once again let me emphasize that Grannyring's issue arises even when the preamp's power cord is not plugged into an AC outlet, and when no components are connected other than the preamp, power amp, and speakers. Therefore I don't see how the problem could be due to a ground loop, because there is no loop in that situation.
The facts that the problem is not present when nothing is connected to the inputs of the amp, but is present when the preamp and interconnects are connected to the amp, would seem to suggest an issue involving emi/rfi pickup. However, as Grannyring indicated in his thread that I linked to earlier, in addition to its normal RCA and XLR inputs his amp provides separate RCA and XLR inputs that are processed through high pass filter circuitry. And when the preamp is connected to the normal RCA inputs the problem exists even if any of the other three inputs are selected on the amp. Which leaves me totally baffled, unless perhaps something is defective in the amp's input select circuitry. But that, in turn, would not seem to explain why the problem wasn't present with the previous speakers.
Vett93, regarding your otherwise excellent posts it should be pointed out that with many designs isolating signal ground from chassis and AC safety ground may involve considerably more than minor modification. For starters, the ground sleeves of RCA connectors may not be isolated from chassis. Also, it would seem possible that in many cases changing the component's internal grounding scheme might result in unpredictable sonic side-effects.