If the preamp volume is turned all the way down, you shouldn't be hearing anything at all.
I think these will be the most likely causes of the noise.
Preamp
Amp
Gound loop
Bad speaker driver
If the noise is only on one channel you could swap speakers to see if the noise follows or if it stays. You could also try to hook-up other speakers to your amp. Otherwise you will need to remove/swap components from the system one at a time to isolate the problem.
I think these will be the most likely causes of the noise.
Preamp
Amp
Gound loop
Bad speaker driver
If the noise is only on one channel you could swap speakers to see if the noise follows or if it stays. You could also try to hook-up other speakers to your amp. Otherwise you will need to remove/swap components from the system one at a time to isolate the problem.