Theoretically a 5 amp socket can handle about up to 1200 watt devices . Practically it isn't good trying to use the maximum capacity of the socket. The problem with the socket is contact resistance. It will start heating up with increasing current and if it's very high current it will eventually fail.
More than that it will cause the mains voltage at the equipment to fluctuate with fluctuating current flow like in an amplifier playing music. Some systems might not like that at all !
To ensure minimal loss at the power socket it might be better to use a 15 amp socket as it has more contact surface area leading to lower contact resistance.
However quality of the plug and socket is very important too.
For smaller devices ( other than power amps) like DVD player, preamps etc. the current is very low as they are very low wattage units. They will do OK on a 5 amp socket.
Note that a 15 amp socket is only worth it if the power cabling to it is also 15 amps capable .
If I had control, I'd lay wires for power and earth separately from the central distribution point without anything else on that power line , not even light !
As far as possible the Earth line must be separate without other connections on it.