While I agree that many bass players use 10" speakers, they use at least four of them (often eight!) and the ONLY thing those 10" speakers have to do is the bass part!
The trouble is, most PA's sound good ONLY up to a certain volume level. Then really bad things start to happen - distortion, non-linearity, failure..... Instead of deciding what is the LEAST you can get away with, if you have any ambition to play larger rooms or less background functions, you need to decide on what is the MOST PA you are likely to need. Or one day you will be offered an outdoor party of 500, and be inaudible...
And a system that sounds good at 50% doesn't necessarily sound good at 75%.... It all depends on what you are putting in and what level the power amps are running at, the knobs are there for entertainment purposes (these go to eleven!). Just because your car's speedometer SAYS 150mph at the end of the dial doesn't mean it can get there safely!
So don't buy toys unless you are SURE you'll never need more....
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An arranger is just a tool. What matters is what you build with it..!