Speaker Oscillation issue ?

Hello All...

This is a problem that has been persisting for quite some time... I noticed my woofer on my right Harbeth 40.3XD was acting a bit strange . I turn off the music & the woofer is actually still moving in / out with no music? I tried moving the outputs to see if the problem follows to the other side and also swapping all tubes , no change....must be the amp acting up huh? Any advice appreciated...

Here's my gear : 

Manley Monoblock Snappers

Manley Jumbo Shrimp Preamp

Harbeth 40.3XD Speakers