Hum problem - help

Hello guys, I am having a bizarre hum problem that is driving me crazy. I am using Levinson 33H monoblocks with B&W 800 speakers. The left channel has this constant hum coming from the midrange and tweeter. I naturally thought it must be the amplifier so I switched the monoblocks (the left one got moved to the right speaker) but the hum remains in the left speaker. Then I switched the interconnects on the preamp (left channel interconnect going to right channel amp and so on) and still the hum is coming from the left speaker. How can this be and what can be causing it? No matter what I try it doesn't go away. Any help would be appreciated.
Hum is sometimes difficult to track down. I had a hum problem in my right channel just recently and the only thing that made it go away was using a 2-prong cheater plug to plug in my DAC. And the results were the same using 3 different DACs so it had to be an odd ground loop of some sort. Good luck!
How about switching your speaker cable?
Also, try the following:

1] Grounding just your preamp and using cheater plugs for everything else.
2] Reversing the AC prongs on ONE amp only. If not better, then the other.
3] Try unplugging all source RCAs from your preamp. If hum gone, add back individually to see which is causing hum.
4] With grounded pre, grounding one amp and not the other. If not better, reverse process.
5] Lifting all grounds.
6] Checking that amp chassis is not in contact with metal shelving.
7] Check that ICs are not in contact with metal shelf postÂ…this caused a nasty hum in one of my channels recently.
If all of what Bojack suggests above fails, I believe I'd be scrutinizing the crossover in the B&W's...or the wiring to them.
Connect a ground lead from every Amp to your Preamp.

Like a "Y".