You say that the hum is present with the pre in standby - this would tend to exonerate it, and everything before it. Try disconnecting the input leads to the power amp, then turn it on - if the noise is gone, it may have had something to do with the interconnects here. (Make sure the cables are not in close proximity to AC cords, then swap the leads right for left - if the noise moves with the cables, suspect a faulty wire/rca jack connection.) If the noise is still present, turn off the amp, reverse the speaker leads right for left at the amp, and power it up again - if the hum stays in the same speaker, you may have an AC cord too close to that cable. If the noise moves to the other channel, suspect the amplifier itself (possibly its input jacks). You could try using a substitute amp (borrow if necessary) to check this. (It is unlikely that a noise you describe as a "hum/buzz" could originate in the loudspeaker itself [driver, crossover], so its seeming presence at the tweeter only could point to the speaker cable in a biwire setup, but the super-steep crossover on this model could seem to produce the same effect when fed a noise signal above the crossover frequency.) And if all this sounds like too much hassle, well, this is what dealers are supposed to be for!