Speaker Hum Question

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!

In my previous post, I realize, I may not have given enough consideration to the description "dual-mono" of the amplifier. I am not familiar with this model, but if it is truly thus - that is to say, all the way back to the wall with 2 power cords - then grounding issues could indeed be in play, even with the noise present in only one channel. Best of luck!

I had this identical problem with a counterpoint SA 1000 preamp and it turned out to be a bad tube.

Andrew, If your preamp is in standby then its not the preamp.

The focus should be the amplifier. You mentioned that your amplifier is dual mono.
It is possible that the slight hum you hear is due to the output transformer exhibiting vibrations from your AC source. That would explain the reason why you are only hearing it out of one channel and not the other. Its not a big deal at all from the way you described it, but if you feel like playing detective here is what you can do.

listen during different hours over the next week or so. See if the hum goes away during late hours. If it does then its the AC coming into the outlet. You can also try plugging into a line conditioner or a grounded outlet.

Either way I would not classify this as a problem.