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)
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.
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.