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.