Ground Loop Hum with balanced XLR cables only

I am (was?) battling with phono 60 hz hum in my system, too. Spent a lot of time swapping out components, star grounding, floating, relocating, cleaning connections, etc, with no luck. Yesterday may have been an epiphany (not 100% certain just yet). I think it may be a defective IC (RCA). This particular IC was previously defective (humming and shorting out; OMG the loud, crude noise it could cause if wiggled the wrong way), and I had it "repaired" at the factory. I reinserted it in my system concurrent with upgrading my phono stage, got the hum from the get-go, and assumed it had to be the phono stage I had just acquired. Finally, yesterday, I swapped out the IC with an identical model of the same manufacturer and the hum appears to be gone. Now, to be certain, I need to reinstall the aforementioned phono stage and see if the hum remains gone (I had used a full function pre to do my most recent sleuthing, which also had hum, so I figured the phono stage is not necessarily the culprit). My belated point is this: In my experience, ICs can and do go bad, and can cause hum or just plain short out. They are, IMHO, part of the investigative process and should be evaluated and tested when tracking down a hum.