Use a reputable amp capable of providing at least 600 watts. If you hear distortion from the speaker, turn it down a bit. Make sure the amp is installed in the correct direction (horizontal vs vertical) for the make/model you decide to go with. Also ensure it receives proper ventilation to keep it from going into thermal protection.

NO! There is a reason circuits are fused at a particular amperage. It is to protect other devices down the line. You need to find out what caused the fuse to blow in the first place. Use fuses other than what is recommended, and you'll be replacing more parts than what you want.

You definitely don't want to use 14 GA wire with a 20 A circuit breaker. The maximum current a 14 GA wire should carry is 15 A and you have the potential to exceed that with a larger breaker. This is against code and is hazardous. A 15A breaker normally uses 14 GA wire but you can go to larger wire. A 20 A breaker normally uses 12 GA wire, but again, you can go to a larger wire.