Birdman, the "rain" would appear to be just a light drizzle. The APA150 is rated in compliance with the FTC power amplifier regulations. The theoretical maximum increase between an 8ohm and 4ohm rating is a doubling into 4 ohms, but this is almost never achieved in practice. More typical results are around a 50% increase, which would imply about a 50 watt 8ohm rating. 50 watts would be used for a peak of about 101dB, rather than 103dB with 75 watts.
It's unlikely that these slight power differences would be appreciable in your bedroom setup.
