Hum Coming Through Amp, What Happened?

I've been running a Primare I21 amp for over a year now, and have been quite happy with the fact that there is no audible hum that comes through the speakers at any volume output. Out of the blue a couple of weeks ago, a nasty hum has started to emit from the amp through the speakers, audible starting at 20-25% of the volume output and louder as the volume increases. I've changed AC receptacles to a Cryo Hubbell one, I've upgraded my power conditioner, and I've switched the stock power cable to a Magic Power cable (which makes the hum even louder).

I am only using 2 of the 4 inputs on the amp, CD and Line 1 (turntable). The hum only occurs on the inputs that have something plugged in to them, and the hum goes completely silent when switched to an input that doesn't have anything connected to it. I've tried rolling the other components to the other inputs (the hum followed the components), and have tried the upgrade power cable on both of the components (no change). The hum occurs whether the components are off, on, or in standby.

What happened here? How do I make this hum go away? It's driving me crazy! Is there something inside of the amp that can be replaced/upgraded to kill this problem? Thanks in advance!