The Infinity ILs were a pretty good speaker, so unless you are looking to move up a different level of speaker, I'd keep those. Can't really say from your descriptions, but it almost sounds like that sub has a problem. How is the sub connected to the receiver? Line level (RCA) or speaker wire?
A new sub may not fix the issue with the hum, but either way, I'd still probably upgrade the sub. Would need to know about how much you are looking to spend?