Hum in left channel - Parasound Z

I'm using a parasound Z amp and a z tuner at work. I do not have a preamplifier, so I have been using the gains on the back of the z amp. It works fine, but it's a pain to have to reach to the back side of the amp to turn volume up or down. Plus, matching levels by channel is difficult.

I obtained a 10k stepped attenuator that I wanted to use as a temporary volume control. It's stereo as I didn't want to adjust each channel independently.

I tried the attenuator at home last night with a different system. It worked perfectly. I brought it to work this morning, connected everything, and it worked great. However, about 30 seconds in, I developed a hum in the left channel. Now, I can't seem to make it go away. It's a 60 Hz hum. I've swapped speakers, and determined it is the amplifier. I've also ruled out the source.

My ultimate question is, is it likely that I damaged the left channel of the amplifier with the 10k attenuator? If so, is there something obvious that can be replaced in the amplifier to make the hum go away? The left channel operates correctly, just has a hum - which is annoying.

Thanks for any input you can provide.