I use a 20W charger form my android phone. Is this OK for charging the current model wireless-headphones?
The wattage only defines the max that the supply can supply to the load. How much current is determined by the load. You could hook up a 100 amp (500 watt) power supply to your USB device and it would only draw what it wants. In fact, during my Christmas light show, I have a V2 camera that is hooked to a 70 amp power supply.
70 amps at 120 volts is 8400 watts. Perhaps you meant to say 70 watts (which would be 1.7 amps)? A 70 amp cable would be rather thick - your house probably has a 200 amp service, and most outlets in a home are 15 amps max. Even an electric clothes dryer is 30 amps. So 70 amps is a big wire and not normally found in residential wiring. I had a 220v welder that was 50 amps. The plug was huge.
No, 70 Amps at 5 volts - 350 watts (about the size of a brick).