Fans are replaceable but you have to get one that is equal to or better than what the fan in it puts out. The hum could be anything, though, and it isn't necessarily always in your amp. Could be something weird in your house. But I'd say clean it with Deoxit, especially the effects loop jacks, and can't remember if there's a tube in it or not but if there is, try replacing it. Beyond that, try to eliminate other causes that could be coming from your home or your bass. What kind of bass, anyway? Single coils or humbuckers?
Ampeg Portaflex Club #1