damaged capacitor and/or power amp
It sure looks blown to me.

You should definitely try just replacing the capacitors. There look to be two capacitors of the same size/model. Considering that one failed, the other is likely to fail too. (bad capacitors tend to come in batches)



The cause of the failure is harder to guess. Capacitors do tend to fail over time in hot environments, so it doesn't necessarily indicate a failure in other components, though it's not possible to completely exclude the possibility of another component causing the failure.

The best thing to do would be replace all the big electrolytics, and power the amp up on a variac with something that lets you monitor the current draw. If there is another failure, the amp will draw a lot of current.  If you're quick about it, It's generally possible to determine if there is another problem (which could damage the caps), and turn off the power before they're actually damaged.

This is a lot harder in semiconductor amplifiers then tube amplifiers, as semiconductor (e.g. transistor) amps tend to fail much more precipitously. Tubes are nice, as they are much harder to damage, and typically give visual warning that they're in the process of failure.