I'm evaluating compact vocal/guitar amps to perform in small venues - nursing home lounge, church social room, etc. I'm confused about watt ratings. Here's a sample: Roland Cube 6.5" speaker, 30 watt Fender Passport Mini 6.5" speaker, 7 watt Behringer B207MP3 6.5" speaker, 150 watt User reviews suggest all are adequate for my intended use. They all have the same size speaker and are about the same price (within $30). So why are the watt differentials so big? How can a 7W amp compete with a 150W? Thanks for any explanation you can provide.