Loud thud heard through speakers

Noticed a weird sound while listening to music today. A loud thud through the speakers during loud vocal passages. I'm not sure what's causing it but happened twice and on different records. One was Muddy Waters Folk Singer and the other Dire straits Brother in arms. 

Amplifier: Graaf GM50BSpeakers: Wilson Benesch ACTSubwoofer: Sumiko S9
At this time I'm not sure of it's the Sub or the Speakers. Did anyone experience anything like this before?