Popping noise when muting pream

I have a VTL preamp hooked up to a sonic frontiers sfs 50 amp using all nordost cables. I noticed a hum in the right channel and when I mute the preamp it pops.
I've checked all the tubes and they are seated fine, I also checked my cables and they are also seated.
My equipment is plugged into a Panamax power conditioner. Suggestions?
I had that problem twice with different BAT preamps, both times it turned out to be a leaky capacitor. It sounds like your preamp needs some professional service.
DC offset at the output of the preamp and then muting it could cause a pop. Also if you had a faulty ground connection inside one of your interconnects or RCA jacks that could also cause the hum. Swap the cables and see if it moves to the other channel.
Lift the ground that is from the tone arm away from the chassis ground on the phono pre-amp. You could have the classic ground loop problem. If this does cure the problem then look for a leaky cap.