sorry guys, just to clarify, i don't really bother about what the theory says but in reality, it's not possible to destroy speakers just because you are using an amp that has a higher wattage than the wattage rating on the speakers.
the wattage rating is not an indication of what the speaker can handle. it is an indication of what the speaker can produce. in the case of 300W amp driving a 200W speaker, the speaker isn't capable of running 300W. period. all you are ever going to get from the cab is 200W, so the amp is never gonna destroy the cab that way.
the analogy with the pipes helps us understand what happens when the resistance of the cab is higher than that of the amp, but it doesn't explain the consequences of when it is lower.
the resistance rating on the amp refers to the maximum load that the amp can handle. the resistance rating on the cab refers to, well, the resistance of the cab. the resistance of the cab should never be lower than the minimum resistance rating of the amp. when an amp says something like 500W @ 4ohms, 300W @ 8ohms, it usually means that 4 ohms is the MAXIMUM load that the amp can handle. anything more than that (ie lower resistance) and you will damage the amp, not the cab.
the only way that the cab can blow is if the output (loudness) is more than what the speaker can handle. if you encounter enough amps you will realise that even at the same power rating, some amps are louder than others. different amps also have different capability of running at maximum performance. it don't think there is an amp out there that will necessarily become louder when the gain is at 100%, the amp is bound to distort.
it is possible that you can blow a speaker if you crank an amp that has a power rating that is higher than that of the cab, but it is not necessarily the case. if lets say your bass has a hot active preamp, or if you are running a boost or OD in your chain, different amps may have different amounts of headroom, even at the same wattage rating. so even if your amp is perfectly matched to your cab, it is still possible to blow the speaker.