First go to Autozone and borrow or buy a mechanics stethoscope. It is basically a set of earphones connected to tubing which connects to a probe. Cost is like $10. This will help you localize the hum to a specific area of the amp. The cover is held on by several screws and when they are out you can just slide it off the top and remove it. Make sure the amp is unplugged for an hour or so before you take the cover off. Once the cover is off do a visual inspection to see if it is something obvious. If nothing is found then use a nonconductive probe to genttly push around things like heat sinks circuit boards and other hard mounted items. Don't poke at anything soldered to the circuit boards and do everything gently. If you want to see what it looks like before you remove the cover there are some pictures of the amp without the cover on the forum somewhere. I will try to find them.

With your system off, disconnect one channels input and output and then turn the system on to see if the hum is still there. If it is turn the system off and then move to the next channel.