Assume this is what you are talking about:
http://www.provantage.com/apc-h15~7AMPS032.htm
It is 1.5 KVA = 1500 watts = 120 volts at 12.5 amperes current.
Power = volts times amperes= watts
Wii, PS3, Xbox360, Cable Box , DVR -fairly low power total < 2-3 amps
Yamaha 663 = 665 watts = 5.6 amps
Emotivia XPA-3 = up to 900 watts = 7.5 amps ( it is actually probably not this high)
URC MRF-260 = very low ((9volt transformer) probably around .3 amps.
Just your Yamaha 663 and Emotiva XPA-3 can be 1.5 KVA themselves; but that is with both at maximum power draw, which is probably unlikely.
The way UL (electical Underwriter's Laboratory) rate devices you probably have at least 10% more capacity on your H15 and could probably run everything on one. Also the power specs on the Yammy and Emo are the max and actual run current would probably make your total pull below the limit.
However, it would probably be more prudent in the long run to buy two H15s and run the Emotivia on one and everything else on the other ! One other consideration the H15 looks like it is match in power for the power of a typical single outlet house utility circuit.