Quote Mike Stranks:
Don't forget you can use two adjacent (correctly wired) sockets to give you 2 x 13 amps.
Be very careful here, while it might be true for two separate sockets provided there is not too much else on the ring (Watch out for caterers), a standard double 13A socket is only actually rated for 20A total load in most cases (The MK Metalclads are a notable exception).
Now the thing is that fuses (and up to a point circuit breakers) respond to RMS current, so a short term transient will be unlikely to case a problem, thus you can run far more then 3KW of amplifier off a 13A socket without any real problem AS LONG AS THE RING IS NOT ALREADY ON THE EDGE.....
By the time you hit 500 capacity rooms you should be safely clear of lighting rigs running from the ring main and the worst of the sort of 'caterer' that plugs 3 tea urns and a toaster into a 4 way, but being aware of things like large smoke machines which can pull a lot of current is a good idea.
TBH at that level there should be a dedicated 16, 32 or 63A supply available, I would look at a small distro board as being a worthwhile investment.
Audiophiles use phono leads because they are unbalanced people!