I just spent the last 2 1/2 hours figuring all this out. Look at my sig. I decided switched the 7500 on the mains with the 2200's. The hum went away. I switched outlets on them but it didn't matter, the 7500 hums but the 2200's don't. The 2200's no power each LSi15 (not bi-amped). While I don't get a hum, I do get a hiss from the tweeter, but nothing from the mid-range drivers.

so, did something go bad in my 7500???