Have you tried a cheater plug (3 prongs to 2 prongs) on the Bel Canto's power cord? This would rule out/rule in a ground loop problem.
In addition to Rich's comment, you can determine the source of the hum by disconnecting everything from the amp (when its turned off!) except for speaker cables and the speakers. Turn it on, no hum? Good. Turn it off, plug in IC's from pre amp (with all other IC's disconnected from the preamp) - same scenerio as above, til you find the source of the hum.
Also put you ear next to the amp - you might hear a 60cycle buss from the transformer. Put you ear next to the midrange/tweeter and see if you hear a hum.
Lastly, make sure your IC's are not lying parallel to power cords or power sources, transformers and the like. Make sure that you don't have stuff with transformers near your pre-amp.
In short, though its too late for this post, take your time and be methodical in your approach.
If you determine that the hum is definitely coming from the amp or pre-amp, it might very well be a DC problem. I had this happen with a receiver and it nearly drove me nuts. Tried all sorts of tests and solutions and finally bit the bullet and bought a PS audio Humbuster which solved the problem instantly.
Before I put in a dedicated line and really solved my ground loop problems (which I had been masking with a so-called ground loop interrupter), I had intermittant hum/noise directly from the eVo6 chassis when certain halogen or fluorescent lamps were used on the same power line. It usually went away with lamp recycling.
In my other house, which had better AC all along, the eVos are just fine.
In your case, it could be DC or RF-induced noise on the AC lines.
Thorman may have your answer.
If you have cable tv, try to take the cable wire out of the first point of entry into the system, which is probably a cable box. But remember, disconecting the cable box would not show anything. Only taking that cable wire out will tell you if it's the problem. If your amp hums after that, it's something else. But if it's the cable tv, musicdirect.com sells this little thing called Mondial Magic Box
My amplifier was humming and it was the cable tv, so I bought it at music direct. Problem solved....Good luck.
i've got one for ya if anyone wants to diagnose it. I have a VTL monoblock with an intermittent hum. sometimes it is preceeded by a volume spike (in a monoblock that is supposedly run wide open whith volume controlled at the pre...and it follows the amp so not a pre issue). the hum always goes away if you toggle the power switch (with tubes you don't even hear the toggle effect on the music the hum just goes away). sometimes it will go away if the other amp or the pre is toggled. sometimes it can go away...or start again if you wiggle the audio input at the amp. i wish the darn thing would break so i could send it in for repair. shipping for a checkup is pricey