I know this sounds silly but my subwoofer [Vibe Alpha One] has started vibrating. This tends to happen after a reasonably loud explosion. I get the normal rumble and all that but immediately after, there's an annoying vibrating sound that lasts a couple of seconds. I guess the best way to explain it is, say you put an empty cup with a teaspoon on top of your sub and listen to the sound the teaspoon makes when the subwoofer naturally vibrates. Don't know if it's the way I have it set up because I've never noticed this until a day or two ago. The sub is about 3 metres from the amp in a corner and I have it hooked up to a Pio 912 by a single cable [5m Profigold subwoofer cable] to the lower left input on the sub. Phase is set to zero, crossover is turned all the way clockwise and volume is at about one third. The crossover on my amp is set at 100hz [the only other options are 150 and 200 and I've tried both]. The sub volume on the amp is -3db although I have tried the volume at different levels. Does anybody have any ideas as to what is causing this, like a poor quality lead or the sub being damaged? Sub is only about 6 weeks old so is well under warranty. My other speakers are MS902 fronts, MS905 centre and MS302 rears all set to small on the amp.