I used two iPhone SPL apps. There were fluctuations in readings from both apps, but they were consistent enough to determine I've got about an 8dB loss from 1m to 4m (4dB loss per distance doubled).

Back to the gist of my first question. If I have a 5.1 system and each speaker requires 100W to reach reference level (105dB peaks), do I need an amp/receiver cable of 100W per channel, all channels driven?