You can safely match 100w speakers with 200 watt amp, since your likley to go deaf before breaking them unless you live in a castle.
A much greater risk is using a low powered amp with insensitive speakers, since an underpowered amp will clip the waveform when it reaches peak power, This clipped waveform contain high energy HF which usually results in molten tweeter voicecoils.
A 200 Watt amp would cost a lot though wouldnt it? I like Cambridge Audio amps but which Cambridge amp would be the suitable for a pair of diamond 9.1s? Im talking for my bedroom about 4 metres in length and 2 metres wide.
any of cambridges current amplifiers are perfectly suitable for the 9.1's unless you want to hear it extremely loud continuosly, or you have the speakers in a particularly large room (greater than say 30 ft square), in which case you would need a more powerful amp for the former, and probably a more powerful amp, and bigger speakers for the former.