Humming sound from my speakers

When I turn on my power amp there is a loud humming sound from my speakers. Although the sound gets softer after about 3 seconds there is still an audible hum. I don't think it is my peramp although when I adjust the volume through the pre amp you can hear a crackling sound from the speakers

My power amp is Audio Research VT 100 (tubes)and my pre amp is LS 15 (tubes)

Any assistance would be appreciated.

Thanks
Daniel