RMS is an average. It is possible for instantaneous power values to be greater than the RMS rating. In fact, they have to be unless you're sending a constant non-varying signal, which would, at least, be very uninteresting to listen to, amplified or unamplified. If the fuses are set at the amp's RMS value (divided by voltage, obviously), then normal usage could easily blow the fuses as the waveforms approach their peaks.