Strange noise from speaker at power-off

For several weeks now I've been getting a strange noise from the left speaker when I turn off the left monoblock amp. The amps are the Rogue M-180 monoblocks which are tube amps. The sound is a staccato popping sound (getting faster) from the woofer that then turns into a high pitch from the tweeter, all for a few seconds. So far no damage to the speaker, but it sounds rather disturbing. At first I thought it was a tube going bad so I put the original KT-90s in place of the KT-120s I've had for a few years now. The noise was still present. The bias looked fine with the KT-120s (see below for the bias issue with the KT-90s). Nick from Rogue suggested it could be a blown fuse, but I opened the amp up and the fuses look fine. Also, there was no problem with powering the amps up, and the sound was fine as well. I haven't swapped the input and driver tubes between the left and right amps yet, but I think it'd be rather unlikely if the noise was generated by those tubes. Or maybe I'm wrong? 

Another problem that may or may not be related is that when I swapped the power tubes, the bias meters on both amps simply stopped working. When I flipped the toggle switch on the potentiometers, the meter hand didn't even move; for all eight tubes (M-180s allow you to bias each tube individually). The strangest thing was that the sound was absolutely fine! 

Today I put the KT-120s back and the bias meters appear to be working ok again, which was fortunate as the bias was all over the place. 

Advice? :)