On the other hand, if you gave us a huge amount of information, including brands and model numbers, we might be able to help.
Some basic ideas, though, include this: Is the hum always present at the same volume, or does it go up and down with the volume? This would indicate whether it's getting injected into an input or is more of a power amp situation.
Measure resistance between the ins and outs of every conductor of your baluns. It should be infinite. Many things called baluns are not baluns (look them up) but simply wiring adaptors. Wiring adaptors would not isolate grounds and could therefore contribute to hum instead of arresting it.
You say you're "working on" this system. Is all interconnection of everything done? I once had a client who had two surround systems in an apartment, with a common wall between them. He moved to an apartment where the systems ended up forty feet apart, so I ran harnesses from one room to the other.
At the end of one day I had been able to give him basic TV watching capability and there was hum, audible in the speakers and visible as a crawling bar on the TV... but the wiring was not finished. He wanted me to fix the hum right then and I told him we should wait until the system was done. Next day, I added the rest of the cabling and there was no longer any hum.
Hum had been caused by some ground loop(s) in the partially-wired system. When I actually ADDED more grounds by finishing the wiring, the ground loop problem solved itself!
If it comes to someone coming out to help, it might be nice for us to know what area you live in.
A good answer is easier with a clear question giving the make and model of everything. "The biggest problem in communication is the illusion that it has taken place." -- G. “Bernie” Shaw
