Hello, Scary thing happened at practice last night! Thought i would have a go at running my 210 on its own with my ashdown abm500 amp. All was ok until: There was this almighty continous bass frequency, the vu meter went right up and the speakers were vibrating so violently that i thought they were gonna blow, it was like some weird feedback! it happened twice, then i switched off, plugged the 15 back in and it was all fine, any ideas what would cause this? scared phil!