This helped with me in getting rid of the 60hz hum I was hearing with two of my 9440A's.
One I used as my sub amp running a Contra Bass sub,
The other running my second set of mains, JBL 4435's
The worst was the hum coming from the sub amp.
I found that if I made a run of quad shield RG6 with RCA fittings on the cable from the source to the amps and used an adapter to the 1/4 inch plug on the 9440A's, it went away. Occasionally I found it would return after I moved the amps and ended up bending or breaking the cable ends when I push the amp too far back pushing and forcing the cable to bend too much.
I never got around to going from the RCA signal out to XLR in.
I fixed a nightmare problematic sub hum one day years ago after an install I did for a health center. The sub would hum here and there for no apparent reason and at times the hum would go on and off several times a minute.
Then one day during a quiet time, the sub started to hum a little then a lot and then it blew up. The lady described it sounded like a body hitting the roof. After I got done asking her how did she know what that sound was, I figured it out.
Somebody near her place, she was out in the boon docks, was using some sort of high powered CB or similar that was in a car. The people there said the car had several long antenna mounted all over the car. This time the car drove, down her road and the people inside the center heard the car. Then the speaker started to hum and blew.
So all I had with me at that time was RG6. I made a run of it and no hum.
Been using it since.
All kinds of ends and adapters are available for the cable too.
I had a pair of RCA shielded cable that I could hear a hum. Replaced them with the RG6 and I couldn't hear the hum.
I love my 9440A's also.
The Altec Dealer on Maui once told me that it was basically a bullet proof amp and could run 200 watts through a screw driver without damage. Sounded like a story of experience.
I have sent most of my 9440A amps to Iratec for a check up and cleaning.
He knows them inside and out.
Ira Tec Pro Audio's Homepage

Give the RG6 a try running your signal lines from the processor to the amp. Who knows, it was a lot easier and cheaper than having me send my amps in to be checked out or chasing ghosts.

hope it helps
Steve