Calling BAT amp/preamp owner

Hello,
First of all, my set up is vk75se, vk50se, krell kav300cd, and sf grand piano. I just notice yesterday (I didn't really pay attention to this before) that my system produces quite loud popping noise(static?) from speakers when I mute or turn the volume to zero. I am wondering if my fella bat owners experience this too. I tried to pin point the problem but got some mixed results. First, I suspected a bad driver tube, because the popping noise appears on the same side as the bad tube is (I swapped tubes left to right vice versa), but when I use my Krell HTS preamp, this popping noise is not there when I mute/turn down vol to zero. Which mean the driver tube is ok. I'm confuse...