Hi,
i don't have any experiences yet Both K210 and K212 is a 400Watts cabinet. I have ordered it yesterday with a K212, i think it could handle better my 5 strings.
As someone mentioned earlier it's an 8Ohm cab (both of them), which means the amp unable to fry it At 8 Ohms the cab delivers about half of it's wattage. If i would have two K212 (or any 8Ohm cabs though...) with the BQ500 then it would be possible. But only if you crank it up fully. But that level the cab should clip which you hear and then you should lower the volume/power of the amp.