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!
Try putting a cheater plug (a 3-prong to 2-prong adapter, leaving the safety ground prong unconnected) on all of the source components, simultaneously (but not on the amplifier). When you do that, try both connection polarities of each of the source component plugs.

If you don't have complete confidence in the integrity of the shield connections of the interconnects (for instance, if one of them may have been stepped on or tugged recently), try different interconnects.

If they are not already, try connecting the ac power plugs of the amplifier and the source components such that they are ALL connected to outlets on the power conditioner that are not isolated from each other in any way. Alternatively, remove the conditioner altogether and connect all power cords to the same duplex outlet or power strip.

If none of that helps, I would suspect unfortunately that the amplifier has an internal defect in its input switching circuit.

Hope that helps,
-- Al
Interested to see what/if something changed anywhere on the electrical system since the hum started, ie, new ac system, added a cable somewhere else, new LCD in soundroom, etc.