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.

Sounds like ground loop hum. Try using a cheater plug to lift the ground on the three prong plug or use a device designed especially to eliminate ground loops. I'd try the plug first to see it that's the problem.
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From what you describe, this isn't a ground loop, as a "crackling sound" is not a ground loop symptom. The problem is likely your preamp's output. I would sub another preamp into the system and see if you get the same effect.
TURN DOWN THE HENDRIX, that's guitar feedback! Just kidding... I'm with Bulldogger recommendation, check for ground loops. Cheers!
Tried all the above suggestions and the humming is still there. It is from the power amp for sure as coming from both speakers. So cannot be a tube.

Any other suggestions please?

I had the same problem and changed IC from preamp to amp. I went from a expensive IC to a cheaper IC and the hum went away. Go figure.